Senate Judiciary Committee Chairman Chuck Grassley is considering issuing subpoenas to President Donald Trump’s former campaign chairman and two FBI officials close to fired director James Comey as part of the panel’s investigation into Russian meddling in the 2016 elections.

It would be the second time the panel has subpoenaed Paul Manafort, Trump’s former campaign chairman. Grassley, R-Iowa, and California Sen. Dianne Feinstein, the top Democrat on the panel, subpoenaed Manafort in July but rescinded it a day later when he agreed to turn over documents and continue negotiating about setting up an interview with the panel.

Grassley indicated Thursday that negotiations over an interview have since stalled.

“All I can tell you is I’m not very satisfied with the fact that his lawyers aren’t returning our calls,” he said.

It would be the first subpoenas for the two FBI officials, Jim Rybicki and Carl Ghattas, after the Justice Department reiterated in a letter this week that it won’t permit the two men to testify. The department had initially declined the panel’s request in July.

Assistant Attorney General Stephen Boyd said in the letter that the overlapping areas of the committee’s investigation and Special Counsel Robert Mueller’s probe had not yet been sorted out.

If Grassley does decide to subpoena the FBI officials, it could increase tensions between Mueller and Congress as he and multiple committees probe Russian interference and possible links to Trump’s campaign. It also indicates that both Mueller and Grassley see the FBI officials as relevant witnesses to the events leading up to Comey’s firing. Comey has said Trump asked him to end an investigation of former national security adviser Michael Flynn and had also asked him over dinner to pledge his loyalty to him.

Committee aides said Friday that the panel is mulling the three subpoenas. They spoke on condition of anonymity because the negotiations are private. Feinstein has said she would agree if Grassley decided to subpoena Manafort, but hasn’t commented on the FBI officials.

In addition to the three men, Grassley and Feinstein had invited Donald Trump Jr., the son of the president, and Glenn Simpson, the co-founder of a Washington opposition research firm that produced a dossier of salacious allegations involving Trump, to testify. Both held closed-door interviews with the panel in recent weeks.

Trump Jr.’s interview with the committee largely focused on a June 2016 meeting involving a Russian lawyer and an offer to provide damaging information about his father’s opponent, Democrat Hillary Clinton.

The House Judiciary Committee has so far declined to investigate the Russian meddling, but the chairman and top Democrat on that panel met with Mueller on Thursday.

The committee’s chairman, Rep. Bob Goodlatte, R-Va., has said his panel will exercise oversight over Mueller as appropriate, and that Mueller should not be impeded by politics.

Goodlatte has also called on the Justice Department to appoint a second special counsel to investigate “unaddressed issues” related to the 2016 election and former Obama administration officials, including Clinton.

Democrat John Conyers of Michigan also attended the meeting.