Brussels ( CNN ) First France , now Belgium and possibly Greece . Where next ? The recent spate of terror attacks and threats in Europe has many wondering what the next target might be and how the danger can be mitigated . Here are the latest developments : . Five Belgian nationals have been charged with participation in a terrorist organization after police raided a suspect terror cell in Verviers , the country 's federal prosecutor said . Three are in custody , and two have been released `` under strict conditions , '' Erik Van Der Sypt said . And two additional suspects are in French custody after they were detained while trying to cross from France into Italy through the Frejus Tunnel , the prosecutor said . The suspected terror cell in Verviers , which included people returning from Syria , planned to target police officers , Van Der Sypt said . Police said they recovered weapons , bomb-making materials and police uniforms . Two suspects were killed in the police raid Thursday , and a third was injured and taken into custody . Authorities have not released their identities . Heavily armed guards filtered through the streets of Brussels and Antwerp on Saturday , a sight the country has n't seen in 35 years . Why is Belgium particularly vulnerable ? The small country is believed to have more suspected jihadists per capita than any other country in Western Europe . Greek authorities say they are cooperating with Belgian authorities on an investigation , and on Sunday Belgium requested the extradition of one of the suspects arrested in Greece . Greek police confirmed that there have been some arrests and are looking for other individuals , but said that none so far were linked to the Belgian plot . Belgian federal prosecutor Van Der Sypt , however , said the Belgian investigation `` gave us enough reason to ask for the extradition of one of the persons that were arrested yesterday by the Greek authorities . '' The investigation revealed that one of the people arrested in Greece could be involved in the plot that was stopped in Belgium , Van Der Sypt said . European counterterrorism agencies are scrambling to assess the potential terrorism threat from people with suspected links to Islamic extremists . As many as 20 sleeper cells of between 120 and 180 people could be ready to strike in France , Germany , Belgium and the Netherlands , a Western intelligence source told CNN . European Union and Middle East intelligence agencies identified an `` imminent threat '' to Belgium and possibly to the Netherlands , the source told CNN . `` There is a tremendous amount of concern over sleeper cells in Europe , '' said a Western official with direct knowledge of the situation . But while there is a `` realistic threat '' to the Netherlands , government spokesman Edmond Messchaert said there is `` no concrete or specific information of an attack in the Netherlands . '' Two additional people whose DNA was found on the belongings of jihadist Amedy Coulibaly are being sought by authorities , according to a source in Paris with knowledge of the investigation into this month 's attacks in the French capital . The DNA of one was found on a charger belonging to Coulibaly , the source told CNN . Coulibaly is accused of killing four people in a siege at a kosher market and gunning down a police officer . Meanwhile , three women detained in connection with the Paris terror attacks were released from custody Sunday , the public prosecutor 's office told CNN . Nine other people will remain in custody for at least 48 hours while the investigation continues . The detainees were held after a series of terror attacks this month that left 17 people dead -- including 12 people in an attack at the offices of the Charlie Hebdo satirical newspaper . Al Qaeda in the Arabian Peninsula , al Qaeda 's Yemen affiliate , claimed responsibility for the Charlie Hebdo shooting . The publisher of Charlie Hebdo says the newspaper will increase the press run of the latest issue , which features a depiction of the Prophet Mohammed , to 7 million . ISIS , the extremist Islamist group that controls swaths of Iraq and Syria , runs large training facilities in Syria and has access to thousands of potential European recruits . Officials are monitoring groups of men who have returned to Europe from fighting in Syria . It is believed ISIS directed operatives to return to Europe to launch attacks in retaliation for airstrikes against the terror group in Syria and Iraq , according to the official . Several European nations , including the United Kingdom , France , Belgium and the Netherlands , are participating in the air campaign against ISIS in Iraq . CNN 's Ivan Watson reported from Brussels ; Holly Yan reported and wrote from Atlanta ; and Tim Lister reported from Paris . CNN 's Margot Haddad contributed to this report .
Neither Belgium nor Greece confirms whether the probe involves a jihadi cell in Verviers . Source : Up to 20 sleeper cells could be set to strike in France , Germany , Belgium and the Netherlands .