U.S. House of Representatives Republicans may subpoena former FBI Director James Comey to force him to testify before a House panel about the agency’s decisions before the 2016 presidential election, a House aide told Reuters on Monday.

In a potential escalation of tensions between rival inquiries into Russian meddling in U.S. politics, House Republicans were prepared to increase pressure on Comey, who has already refused to testify in a closed session.

Comey’s attorney David Kelley, in a letter dated Oct. 1 that was sent to the Republican chairmen of two committees, said: “Mr. Comey respectfully declines your request for a private interview. He would, however, welcome the opportunity to testify at a public hearing.” A copy of the letter was seen by Reuters.

Representatives Trey Gowdy and Bob Goodlatte chair the House Oversight panel and the Judiciary panel, respectively. Both are probing the FBI’s decisions before the 2016 presidential election; neither is seeking reelection in November.

The Republicans’ probe is running parallel to Special Counsel Robert Mueller’s investigation of Russian interference in the 2016 election and possible collusion between Moscow and the Trump campaign. That probe has resulted in indictments or guilty pleas from 32 people.

If the Democrats take majority control of the House in the Nov. 6 elections, the two committees’ probes will likely end. Between now and then, it was not immediately clear what Gowdy and Goodlatte hope to accomplish. The House is out of session until Nov. 13.

Their probe into the FBI’s handling of Russia’s role in U.S. politics and Hillary Clinton’s emails when she was secretary of state has been criticized by Democrats as seeking to undermine the Mueller probe.

Comey, who was fired by Trump, is seen as a key witness into whether Trump tried to obstruct the Mueller probe. Comey has also been criticized by the Justice Department’s inspector general for announcing the FBI’s decision not to propose criminal charges against Clinton over her use of a private email server.

Republicans have said Comey’s handling of the Clinton matter shows he is biased against Trump. The inspector general found that while he acted improperly, he did not exhibit bias.

Goodlatte recently issued a subpoena to the Justice Department after he alleged it had refused to provide certain documents from the Russia investigation.

Deputy Attorney General Rod Rosenstein, who appointed Mueller, has since agreed to meet with Republicans in coming weeks, said a person familiar with the matter.