Former U.S. special envoy for Ukraine Kurt VolkerKurt VolkerVolker met with adviser for Ukrainian firm at center of impeachment inquiry The five Trump officials Democrats want to depose Tillerson appoints ex-NATO ambassador to lead Ukraine talks MORE told House investigators Thursday he warned President TrumpDonald John TrumpSessions says he still supports Trump despite ouster as AG House Republicans voice concerns about White House's impeachment messaging Giuliani consulted with Manafort on Ukraine info: report MORE’s personal lawyer Rudy GiulianiRudy GiulianiHouse Republicans voice concerns about White House's impeachment messaging Giuliani consulted with Manafort on Ukraine info: report Giuliani says some documents turned over by State watchdog came from him MORE that his Ukrainian sources alleging wrongdoing by former Vice President Joe BidenJoe BidenHouse Republicans voice concerns about White House's impeachment messaging Giuliani consulted with Manafort on Ukraine info: report Top Pence adviser was on Trump-Zelensky phone call at center of whistleblower complaint: report MORE and his son Hunter were not credible, according to The Washington Post.

Volker testified that he attempted to convince Giuliani that his sources, including the nation’s prosecutor general, were providing unreliable information, according to the Post, citing two people familiar with the testimony.

Volker also said he and other officials within the State Department warned Ukrainian officials against involvement with U.S. politics, saying their involvement could open them to allegations of election interference, according to the Post.

Volker was summoned after he was named in a whistleblower complaint that led to an impeachment inquiry against Trump as the official who arranged a meeting between Giuliani and Ukrainian President Volodymyr Zelensky. Volker resigned shortly after the revelation of the complaint and a summary of a July call between Trump and Zelensky.

The report came shortly after Rep. Jim JordanJames (Jim) Daniel JordanBiggs takes over as Freedom Caucus chairman Veteran political journalist: 2020 Democrats walk 'tight rope' on Biden allegations Trump allies go on the offensive against whistleblower complaint, Democrats MORE (R-Ohio), one of Trump’s staunchest allies in the House, told reporters outside the closed-door session that “nothing he has said supports the narrative you’ve been hearing from [House Intelligence Committee Chairman Adam] Schiff [D-Calif.] and the Democrats.”

The report comes the same day Volker provided encrypted texts to Congress with Kiev chargé d'affaires Bill Taylor in which Taylor reportedly told other diplomats "it's crazy to withhold security assistance for help with a political campaign” even as the White House has denied it made the aid contingent on Ukraine launching an investigation.