The inquiry comes after a whistleblower filed a complaint Aug. 12 with the Office of Intelligence Community Inspector General over a phone call made by Trump to a foreign leader.
While the details of the complaint haven't been made public, The New York Times reported the complaint is related to Trump and Ukraine. The Wall Street Journal reported that Trump pushed the recently elected Ukrainian President Volodymyr Zelensy, to investigate former Vice President Joe Biden's son, Hunter.
Update 11:15 p.m. EDT Sept. 24: President Trump is preparing to release the whistleblower complaint to Congress as early as Wednesday, multiple sources reported.
The exact content of the complaint hasn't been made public. People "familiar with the situation" told The New York Times the administration was putting the complaint through a declassification process and that it would be available within days.
Trump also said in a tweet Tuesday evening that Secretary of State Mike Pompeo received permission from the Ukranian government to release the transcript of the call.
Update 6:15 p.m. EDT Sept. 24: House Minority Leader Kevin McCarthy responded to the impeachment inquiry, saying Pelosi "doesn't speak for all Americans on this issue."
"Our job is to legislate. Not to continue to investigate something in the back when you cannot find any reason to impeach this president," McCarthy said. "... It's time to put the public before politics."
Update 5:10 p.m. EDT Sept. 24: House Speaker Nancy Pelosi announced that a formal impeachment inquiry against President Trump has been launched.
"Today, I'm announcing the House of Representatives is moving forward with an official impeachment inquiry," Pelosi said. "I'm directing our six committees to proceed with their investigations under that umbrella ... The president must be held accountable."
Original report: House Speaker Nancy Pelosi will announce Tuesday a formal impeachment inquiry of President Donald Trump, according to multiple reports.
A growing number of Democrats have shown support for beginning an impeachment inquiry, spurred by recent reports that a U.S. intelligence official filed a whistleblower complaint against the president last month.
Citing unidentified Democratic officials, the Washington Post, The New York Times and The Wall Street Journal reported Pelosi would back the move to begin a formal inquiry.
"I've said to people, 'As soon as we have the facts, we're ready,'" Pelosi said earlier Tuesday while speaking at The Atlantic Festival. "Now we have the facts. We're ready. For later today."
Pelosi is expected to speak publicly about the path forward at 5 p.m. EDT.
At issue is a summer phone call Trump had with Ukraine's president, which came to Congress' attention through a whistleblower complaint. Trump has insisted he did nothing wrong in the call, but has suggested he raised Vice President Joe Biden and his son Hunter as part of discussions over corruption in Ukraine — despite no evidence of wrongdoing on the part of either man.
Trump announced Tuesday on Twitter that he authorized the release of a transcript of the conversation.
"You will see it was a very friendly and totally appropriate call," the president wrote.
Check back for updates to this developing story.
The Associated Press contributed to this report.