WASHINGTON – House Democrats said late Thursday that they are investigating Vice President Mike Pence's actions as part of their impeachment inquiry into President Donald Trump's efforts to press Ukrainian President Volodymyr Zelensky for damaging information on Joe Biden.

The chairmen of three House committees leading the investigation said in a joint letter that new reports have revealed that other Trump administration officials "may have been involved in the illicit effort to get Ukrainian help for the president's campaign."

The letter cites reports that Pence may have been aware of the contents of the president's July call in which Trump repeatedly pressed Zelensky to investigate a Ukrainian gas company whose board members had included Biden's son Hunter.

And the letter says Pence's absence from Zelensky's May inauguration may have been related to "efforts to put additional pressure on Ukraine to deliver on the president's demands."

House Democrats on Wednesday threatened to subpoena the White House if it doesn't turn over by Friday a host of documents that include any communication Pence's office had about the July call with Zelensky and information about Trump's decision not to send Pence to the inauguration.

The White House maintains that Trump did nothing improper.

"Anyone that looks at the president's transcript (of the July call) will see that the president was raising issues that were appropriate, that were of genuine interest to the American people," Pence said Thursday.

Pence pointed to the fact that Hunter Biden was reportedly paid as much as $50,000 a month to serve on the board of the gas company when his father, as vice president, was the Obama administration's point man in Ukraine.

Pence said that's worth looking into and, he said, Trump believes "other nations around the world should look into it as well."

"I mean, the simple fact is that, you know, when you hold the second highest office in the land, it comes with unique responsibilities – not just to be above impropriety, but to be above the appearance of impropriety," Pence told reporters when asked about Ukraine and Trump's comment Thursday that China should also investigate Biden.

The Washington Post reported Wednesday that Trump used Pence in his attempt to pressure Zelensky, but was not conclusive on how much Pence knew about Trump's effort. The Post said Pence's national security adviser monitored Trump's call but didn't hear anything he felt should be relayed to the vice president.

The whistleblower complaint that sparked the impeachment inquiry by disclosing the call alleged that Trump instructed Pence to cancel plans to attend Zelensky’s inauguration, a detail given in the context of Trump wanting to wait to see how the new leader “chose to act” in office.

Along with their letter released Thursday, Democratic heads of the House Intelligence, Foreign Affairs and Oversight committees also released text messages showing a high level of apprehension among State Department officials that Trump had linked U.S. assistance to Ukraine to an agreement from Zelensky that he would reciprocate by taking steps that would help Trump politically at home.

"I think it's crazy to withhold security assistance for help with a political campaign,” Bill Taylor, the top American diplomat stationed in Ukraine, wrote in one message.

A second Trump official, Gordon Sondland, the ambassador to the European Union, pushed back, writing: "Bill, I believe you are incorrect about President Trump's intentions. The President has been crystal clear: no quid pro quo’s of any kind."

Sondland also said they should stop talking about the matter online, perhaps realizing the implications of such messages.

Trump eventually released the military aid after a bipartisan backlash from from U.S. lawmakers in both parties.

Contributing: Deidre Shesgreen.