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Federal prosecutors in New York have subpoenaed the brother of one of the recently indicted associates of Rudy Giuliani, according to two people familiar with the matter, as they escalate their investigation in the campaign-finance case.

The subpoena to Steven Fruman is the latest indication of prosecutors’ actions since the rushed arrest two weeks ago of his brother, Igor Fruman, and another defendant, Lev Parnas, at a Washington-area airport. Since then, investigators have doled out multiple subpoenas and conducted several property searches, in one case blowing the door off a safe to access the contents, sources tell CNN.

Federal prosecutors told a judge this week that they are sifting through data from more than 50 bank accounts. In addition, they’ve put a filter team in place as they examine communications obtained via search warrant and subpoena, sensitive to material that could be subject to attorney-client privilege because Giuliani, President Donald Trump’s personal attorney, counted Parnas as a client. A filter team is a separate set of prosecutors who are assigned to examine evidence and set aside material that is privileged.

Since the October 9 arrests, federal agents visited the New York home of Steven Fruman and served him with a subpoena from Manhattan federal prosecutors, the people familiar with the matter said.

Attorneys for Steven and Igor Fruman declined to comment. A spokesman for the Manhattan US Attorney’s office also declined to comment.

Igor Fruman, Parnas and their two co-defendants have all pleaded not guilty in the case alleging they funneled foreign donations into US elections.

It’s not clear why prosecutors are interested in Steven Fruman, or what specifically agents sought from the safe. Steven Fruman is listed in US Security and Exchange Commission filings as the vice president of FD Import & Export, the same company his brother ran.

Igor Fruman also appeared to use the address of a property in Woodmere, New York, that belongs to his brother when making certain political donations, according to Federal Election Commission and other public records.

As they pursue an ongoing investigation into Igor Fruman, Parnas and their co-defendants, prosecutors are also investigating Giuliani’s Ukranian business dealings, CNN has reported. Parnas and Igor Fruman for months have aided Giuliani in what he has described as his effort to unearth damaging information about his client Trump’s political rival, Democratic presidential candidate Joe Biden, whose son was on the board of a Ukranian energy company.

In recent days, Giuliani has been seeking a criminal-defense attorney, CNN has reported. He says he has not been contacted by the FBI or by New York federal prosecutors.