(CNN)Justice Department prosecutors have subpoenaed information about some of former President Donald Trump's lawyers and closest advisers as part of their criminal investigation into efforts to put forward fake slates of electors in the 2020 election, a source familiar with the investigation told CNN.

       The subpoena asks a witness connected to a slate of "alternate" electors for Trump in a state that Joe Biden won to provide to a federal grand jury any communications with government employees, with the Trump campaign and with some of the most prominent people around Trump in 2020, according to the source familiar with the probe.

       The elector-related investigation represents yet another aspect of the Justice Department's sprawling investigation into January 6, 2021. That criminal probe has expanded in recent months beyond the hundreds of rioters who stormed the US Capitol as investigators gather information about Trump's inner circle.

       Former Trump attorney Rudy Giuliani, adviser Boris Epshteyn and campaign lawyer Justin Clark are among the list of names the witness was asked about, the source said.

       The subpoena also seeks communications with several other individuals, including the right-wing lawyer John Eastman, who was a key figure proposing blocking Biden's presidential win, and Bernard Kerik, a longtime friend of Giuliani's who worked to find evidence of voter fraud after the election.

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       In addition to any communications, the grand jury is seeking documents related to attempts to certify electors for Trump in one of the states where he lost the popular vote, the person said.

       The New York Times first reported on subpoenas seeking information about some Trump lawyers.

       Representatives of Clark, Epshteyn and Eastman declined to comment. Giuliani's attorney didn't immediately respond.

       Kerik's attorney, Tim Parlatore, said his client would be happy to cooperate with an investigation if needed. "But it's impossible to understand what DOJ would even be looking for as this is a political issue, not a criminal issue," Parlatore said.

       Prosecutors are also seeking information about Trump lawyers Joe diGenova, Victoria Toensing, Jenna Ellis and an appellate attorney who assisted the campaign, Kenneth Chesebro, the source said. Those four people didn't respond to requests for comment.

       The plan to put forward slates of fake electors for Trump during the certification of the 2020 vote was part of a push to get then-Vice President Mike Pence to overturn Biden's election win.

       The subpoena, the source said, specifically asks about the signing or mailing of certificates of elector votes for Trump in 2020.

       Legal experts have said that the Republican electors who sent the fake certificates or anyone who facilitated the plot could be vulnerable to prosecution, including for providing false voting documents to the federal government.

       No one has been charged with a crime related to the alternate slates of electors.

       CNN's Zachary Cohen contributed to this report.