The FBI is seeking an interview with a CIA whistleblower whose complaint led to an impeachment inquiry into whether President Trump abused his office in his dealings with the Ukrainian president, according to three people familiar with the matter.

A special agent with the bureau’s Washington Field Office contacted one of the whistleblower’s lawyers last month, and the FBI and the legal team have traded messages since, said two of the individuals, all of whom spoke on the condition of anonymity to discuss a sensitive matter.

No date for an interview has been set, and it is not clear whether one will be, the people said.

Andrew Bakaj and Mark Zaid, the whistleblower’s attorneys, declined to comment. Spokespeople for the Justice Department and the FBI declined to comment.

The FBI is interested in the “substance” of the whistleblower’s complaint, said one person familiar with the matter. The complaint centered on a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky and concerns that Trump was seeking to pressure Zelensky into opening investigations that would aid Trump’s 2020 reelection effort.

The bureau does not appear to be pursuing a leak investigation, the person said.

The development, first reported by Yahoo News, adds a wrinkle to the political debate over whether Trump’s conduct constitutes an impeachable offense — a question at the center a fiercely partisan fight that the FBI has so far avoided.

Congressional investigators are holding public hearings examining whether the president conditioned a White House meeting and crucial U.S. security assistance to Ukraine, which is battling Russian-backed separatists in its eastern region, on a promise by Zelensky to publicly announce two investigations that would benefit Trump politically.

One investigation involved Burisma, a Ukrainian energy company whose founder has been targeted by corruption prosecutors and on whose board Joe Biden’s son Hunter sat while the elder Biden was U.S. vice president. The other investigation sought by Trump focused on a widely discredited theory that Ukraine, not Russia, stole Democratic National Committee emails during the 2016 election campaign.

The Justice Department determined in September, after reviewing the whistleblower’s complaint, that it saw insufficient evidence to undertake a criminal investigation into Trump for possible campaign finance violations. At the time, spokeswoman Kerri Kupec said “no further action was warranted.”

It is unclear precisely what the FBI is investigating in its request to question the whistleblower, or whether it did so with the approval of Justice Department officials. Justice Department prosecutors in New York are investigating the financial interactions between the president’s personal lawyer Rudolph W. Giuliani and two businessmen who were arrested on charges of violating U.S. campaign finance laws. That investigation also involves figures in Ukraine.

Trump and his supporters have sought to unmask the whistleblower.

“I do not believe that DOJ’s declination decision should or could preclude the FBI from investigating further,” said Chuck Rosenberg, a former federal prosecutor. “If the FBI has information about potential crimes, they have an obligation to look further.”

Bakaj and Zaid have received death threats targeting them and their client. Though they have reported those threats to the FBI, there is no indication that the bureau’s interview request is related to that issue, said one person familiar with the matter.

The lawyers have offered to make their client available to answer congressional investigators’ written questions under oath, but lawmakers have yet to accept that offer.