[George Howell, Cnn Anchor:] The U.S. president says the world cannot be complacent on nuclear security and warns to keep radioactive materials out of the wrong hands. Plus: Donald Trump tries to right his ship after a week of flip- flopping while the Democratic candidates accuse each other of lying. And remembering Garissa University. CNN speaks with survivors of that massacre in Kenya one year after Al-Shabaab militants launched a devastating raid there. Live from CNN World Headquarters in Atlanta, we want to welcome our viewers in the United States and around the world. I'm George Howell. NEWSROOM starts right now.
[Howell:] A good day to you. We begin this hour with the nuclear summit in Washington, D.C., and the efforts to maintain security of nuclear stockpiles. The U.S. president, Barack Obama, says an important treaty is now expected to go into effect. That treaty requires member states to do more to secure their radioactive materials. China pledged to work closer with the U.S. as well, playing a key role in enforcing sanctions against North Korea, which has nuclear weapons. Mr. Obama also says he wants to reduce America's nuclear arsenal further but he also wants to modernize the nuclear weapons the U.S. does keep.
[Barack Obama, President Of The United States:] Just the smallest amount of plutonium, about the size of an apple, could kill and injure hundreds of thousands of innocent people. It would be a humanitarian, political, economic and environmental catastrophe with global ramifications for decades. It would change our world. So we cannot be complacent. We have to build our on progress.
[Howell:] World leaders at the summit also focused on keeping nuclear materials out of the hands of terrorists. But one major nuclear player decided not to show up. Our Brian Todd reports on why Russia skipped this major summit.
[Brian Todd, Cnn Correspondent:] With the Brussels and Paris attacks, a clear message from ISIS. They can and will hit Western cities and kill as many civilians as they can. And it appears ISIS wants to create more devastation. After the Paris attacks investigators in a raid found surveillance footage of an employee at a Belgian nuclear facility.
[Andrew Bieniawski, Nuclear Threat Initiative:] And that nuclear facility had highly-enriched uranium, but also produced these radiological sources. So now we know that they actually are trying to take steps to try and acquire these materials. And so therefore we need specific action coming out of this Nuclear Security Summit.
[Todd:] But at the Nuclear Security Summit in Washington one leader who could make a huge difference in securing nuclear material was a no- show. Vladimir Putin has once again snubbed President Obama.
[Ben Rhodes, Deputy National Security Advisor:] Russia's lack of participation obviously in our view is frankly counter-productive.
[Todd:] Russia has more than half the world's stockpile of nuclear materials and its safeguards haven't always been air tight.
[Bieniawski:] There was an example several years ago where an insider employee at one of the Russian facilities at Luch was slowly taking out small quantities of nuclear material from that site.
[Peter Bergen, Author, "united States Of Jihad":] Certainly historically I mean I've been to Russia and gone to some of their nuclear facilities and, you know, in the years after 911 it was very amateur the way they were, they were holding these radioactive materials.
[Todd:] But experts say the Russians have since gotten better at securing nuclear material. Security analysts say, if Putin had shown up at the summit, he could have shared intelligence on how to keep ISIS away from nuclear and radiological material. Either way, the terror group would have a tough time getting its hands on a nuclear weapon. But radiological material for a dirty bomb, often stored in hospitals and industrial complexes, is far less secure. ISIS can access that. And ISIS supporters in the U.S. haven't been bashful about where they want to strike. This kind of target is really their aspiration, right?
[Bergen:] Yes. So mass casualty attack clearly their aspiration. The radiological bomb, if it went off here, many people dead in the immediate vicinity, but the much bigger deal, Brian, is that it would disperse radioactive material all around downtown Washington, several block area. It would close down the city for maybe years.
[Todd:] Responding to the criticism of Putin for not showing up at the summit, a Russian official told us they're not sending a bad message and there's much more to nuclear security than this summit. The Russians have also ticked off some reasons for Vladimir Putin's absence from the summit. They say some countries with nuclear material, like Iran, are not participating. They say most of the key goals for nuclear security have already been reached, so no need to show up. And they say the U.S. is unfairly pushing its agenda on international groups like the U.N.'s nuclear watchdog and Interpol Brian Todd, CNN, Washington.
[Howell:] America's choice 2016: the current U.S. president had some sharp words for Donald Trump. This after his recent comments that the United States should back out of its security role in Asia. The Republican front-runner suggested at a CNN town hall on Tuesday that Japan and South Korea should develop their own nuclear arsenals. And at a news conference at the nuclear security summit, Mr. Obama said the remarks indicate that Donald Trump should not be in the Oval Office.
[Barack Obama, President Of The United States:] The person doesn't know much about foreign policy or nuclear policy or the Korean peninsula or the world generally. It came up on the sidelines that people pay attention to American elections. What we do is really important to the rest of the world.
[Howell:] Mr. Obama's criticism of Trump comes as the presidential race heats up in the state of Wisconsin with its primary just around the corner. Voting is set for next Tuesday there, April 5th. And it's expected to be a crucial contest for both parties. There are 42 delegates at stake for Republicans and 86 pledged delegates for Democrats. Local officials say they could see the state's biggest voter turnout for the primary in 36 years. That primary isn't looking so good for Donald Trump. He is 10 points behind Ted Cruz at this point. And that is just one part of Trump's bad week. The past few days have been the most challenging of his campaign so far. Our chief political correspondent, Dana Bash, has this report for us.
[Dana Bash, Cnn Sr. U.s. Congressional Correspondent:] When all else fails for Donald Trump, he tries to change the subject, like he did today.
[Unidentified Male:] Ted Cruz was my roommate. I did not like him at all.
[Bash:] Slamming Ted Cruz in a new Instagram video after one of the billionaire front-runner's worst weeks since the campaign began, causing a bipartisan firestorm with these comments when asked if women should be punished for having an abortion if it became illegal.
[Donald Trump , Presidential Candidate:] The answer is that there has to be some form of punishment.
[Matthews:] For the woman?
[Trump:] Yes. There has to be some form.
[Bash:] That, Trump recanted within hours and later added this
[Trump:] It could be that I misspoke, but this was a long, convoluted subject.
[Bash:] But he has not taken back what he said at CNN's town hall, advocating for more nuclear weapons in Asia.
[Trump:] At some point we have to say, you know what, we're better off if Japan protects itself against this maniac in North Korea.
[Bash:] Now Trump is refusing to rule out using nuclear weapons in Europe.
[Trump:] Europe is a big place. The last person to use nuclear would be Donald Trump. That's the way I feel. I think it is a horrible thing. The thought of it is horrible. But I don't want to take anything off the table.
[Bash:] Trump's rivals continued to blast him, including John Kasich, who until this week mostly held his fire.
[Gov. John Kasich , Ohio, Presidential Candidate:] The problem for him with town halls is he actually has to answer questions in a specific way.
[Bash:] Kasich also went after Ted Cruz for having a thin leadership record.
[Kasich:] His record is shutting down the government and making everybody he works with upset.
[Bash:] As Trump sees his unfavorable ratings rise and support among women fall, he's quick to point out that he is still the front-runner by a long shot and that even if he arrives at the GOP convention in July without winning the nomination, if he is close, it should be him.
[Trump:] I really think that whoever has that kind of an advantage should get it.
[Bash:] But the first-time politician is also learning that seizing the Republican nomination takes more than just winning contests. It takes winning over delegates in some states where rules vary. Sources tell CNN that educating Trump about the complicated delegate process was the subject of Trump's meeting this week with Republican Party chair Reince Priebus as RNC headquarters in Washington.
[Trump:] Very actually a terrific meeting, I think. And it's really a unity meeting.
[Bash:] CNN is told that Priebus used the meeting to ask Trump to ease up on trashing the RNC, as Trump did this week at CNN's town hall.
[Trump:] I have been treated very unfairly. I will give you an example.
[Anderson Cooper, Cnn Anchor:] Unfairly by who?
[Trump:] I think by basically the RNC, the Republican Party.
[Howell:] That was CNN's Dana Bash reporting for us. In yet another reversal on Friday, Donald Trump told CBS News that federal laws are set and should not be changed to outlaw abortion. Later, his spokeswoman said Donald Trump was just giving an account of the current laws, which he would change as president. Now to the Democratic race, where things are getting more personal. Hillary Clinton says that Bernie Sanders' campaign is lying about her record and she's sick of it. Now Sanders says that Clinton is the one lying. Senior Washington correspondent Jeff Zeleny has this report for us.
[Jeff Zeleny, Cnn Sr. Washington Correspondent:] Hillary Clinton just can't shake Bernie Sanders.
[Hillary Clinton, Former Secretary Of State, Presidential Candidate:] This is really personal for me.
[Zeleny:] Their Democratic fight isn't winding down, but ramping up and expanding to new fronts.
[Sen. Bernie Sanders , Vt., Presidential Candidate:] Secretary Clinton.
[Zeleny:] Sanders and his supporters keeping alive their criticism of Clinton receiving contributions from the oil and gas industry. This confrontation with a climate change activist going viral.
[Clinton:] I do not I have money from people who work for fossil fuel companies. I am so sick, I am so sick of the Sanders campaign lying about that. I'm sick of it.
[Zeleny:] The outburst offers a fresh window into a rising frustration with Sanders. The Clinton campaign accepts money from people who work for oil and gas companies, not the companies themselves. Sanders called it a distinction without a difference.
[Sanders:] If people receive money from lobbyists of the industry, I think you're receiving money from the industry. And these are not just a little worker there. These are lobbyists who represent the oil and gas industry.
[Zeleny:] But today in New York, Clinton struck back, saying Sanders isn't pro-business.
[Clinton:] I just go crazy when I hear Senator Sanders and the Tea Party Republicans railing against the export-import bank, like it's some kind of evil presence.
[Zeleny:] The Democratic rivals are also tangling over abortion, Clinton accusing Sanders are not properly denouncing Donald Trump's assertion women who have abortions should be punished.
[Clinton:] Senator Sanders agreed that Donald Trump's comments were shameful, but then he said they were a distraction from and I quote "a serious discussion about the serious issues facing America."
[Zeleny:] Sanders cried foul.
[Sanders:] What Secretary Clinton did is take things out of context. I am 100 percent pro-choice.
[Zeleny:] The root of the tension is the length of the race. The Clinton campaign once assumed the race would be all but over by now, as campaign manager Robby Mook noted in this memo after Clinton lost the New Hampshire primary two months ago, writing: "The nomination will very likely be won in March, not February." Sanders has an edge in Wisconsin and is fighting hard on Clinton's turf in New York. He drew 18,000 supporters last night to a rally in the Bronx.
[Sanders:] My father came to this country at the age of 17 from Poland without a nickel in his pocket.
[Zeleny:] Sanders is well behind in the delegate race, but money is keeping him in the game. His campaign says it raised $44 million in March, fortifying it for the final two months of the long Democratic primary.
[Sanders:] Let's take this fight to the White House. Thank you all.
[Zeleny:] But for Bernie Sanders to take this to the White House, he needs to keep winning and winning big. His first target is Wisconsin. He's camping out there all weekend long. Now a top Clinton adviser told me today they believe Wisconsin is basically out of reach. That's why they are focusing so much attention on New York. But judging by the size of last night's crowd in the Bronx for Sanders, that populist streak in New York is alive and well and the Clinton campaign is taking New York very, very seriously. They know a loss there would upend this race like nothing else could.
[Howell:] That was CNN's Jeff Zeleny reporting there for us. Bernie Sanders spoke out about Clinton's finger-pointing at a rally in Wisconsin on Friday. The Vermont senator says his campaign was not lying about Clinton's donations from fossil fuels from that industry and the former secretary of state owes him an apology.
[Sanders:] According to an analysis done by Greenpeace, Hillary Clinton's campaign and her super PAC have received more than $4.5 million from the fossil fuel industry. In fact, 57 oil, gas and coal industry lobbyists have directly contributed to her campaign, with 43 of them contributing the maximum allowed for the primary. And these are not just workers in the fossil fuel industry. These are paid, registered lobbyists. Secretary Clinton, you owe our campaign an apology. We were telling the truth.
[Howell:] Gloves coming off there. Be sure to join CNN on Tuesday for all the day's coverage of this critical Wisconsin primary. It could be a game-changer for candidates on both sides, only here on CNN. You're watching CNN NEWSROOM. And still to come this hour, the deadly bridge collapse in Kolkata, India. A look at who authorities are blaming there. Plus the Garissa University massacre one year on. Survivors reflect on what they saw that day and how they are moving forward. Stay with us.