WASHINGTON – Shortly after his ouster as President Trump's national security adviser, John Bolton told a top House Democrat to investigate the abrupt recall of former U.S. Ambassador Marie Yovanovitch from her post in Ukraine, according to that lawmaker.

During a Sept. 23 phone call, "Ambassador Bolton suggested to me – unprompted – that the committee look into the recall of Ambassador Marie Yovanovitch," Rep. Eliot Engel, chairman of the House Foreign Affairs Committee, said in a statement Wednesday.

"He strongly implied that something improper had occurred around her removal as our top diplomat in Kyiv," Engel said, D-N.Y.

Engel said he released the statement now because of a Trump tweet posted on Wednesday, suggesting Bolton never raised concerns about the Ukraine pressure campaign before the impeachment proceedings began.

"Why didn’t John Bolton complain about this 'nonsense' a long time ago, when he was very publicly terminated," Trump tweeted on Wednesday. 'He said, not that it matters, NOTHING."

Engel said that's not true, pointing to his Sept. 23 call with Bolton.

Engel's statement suggests Bolton played a behind-the-scenes role in the House Democrats' impeachment inquiry. It comes amid a fierce partisan clash over whether senators should call Bolton as a witness in Trump's Senate impeachment trial.

Bolton is now writing a book about his time in the Trump White House. The New York Times reported Sunday that Bolton's book alleges Trump demanded Ukraine investigate a political rival in exchange for foreign aid, the key allegation in the impeachment case against the president. The White House is threatening to block publication of Bolton's book unless the former national security adviser deletes allegedly classified information from the draft manuscript.

The Bolton-Engel conversation came after allegations first emerged, in a Sept. 19 Washington Post story, that an unnamed whistleblower had filed a complaint about Trump's efforts to pressure Ukraine for political favors.

House Speaker Nancy Pelosi, D-Calif., announced the House impeachment proceedings against Trump on Sept. 24, the day after Bolton and Engel spoke on the phone.

Yovanovitch is a central figure in the scandal. Trump ousted her from her post in Kyiv amid a smear campaign by his personal lawyer, Rudy Giuliani. Democrats say Trump and Giuliani saw Yovanovitch as an impediment to their efforts to get Ukraine President Volodymyr Zelensky to open two investigations that would benefit Trump politically in his 2020 re-election campaign.

Engel said he sought to talk to Bolton after he left the White House – to ask Bolton if he would talk to the committee in a closed-door session about Trump's foreign policy.

"I’ve known Ambassador Bolton for years; we have a cordial and respectful relationship and I wanted to thank him for his service," Engel said. "I also wanted to ask if he would talk to the Foreign Affairs Committee, as former Secretary of State Rex Tillerson did, to aid our general oversight efforts of U.S. foreign policy."