President Donald Trump said Monday that written answers from the whistleblower to Congress would be unacceptable — although such answers were fine for the president when dealing with former special counsel Robert Mueller.

"The Whistleblower gave false information & dealt with corrupt politician Schiff," Trump tweeted. "He must be brought forward to testify. Written answers not acceptable! Where is the 2nd Whistleblower? He disappeared after I released the transcript. Does he even exist? Where is the informant? Con!"

Trump was responding to news that Mark Zaid, the attorney for both known whistleblowers who came forward with concerns about Trump's conduct toward Ukraine, said the first whistleblower offered to provide written answers to House investigators to protect his or her identity. Zaid told NBC News on Sunday that he had not yet received a substantive response from House Intelligence Committee Republicans about his offer.

Trump's actions with regard to Ukraine, which included placing a months-long hold on military aid and asking Ukrainian President Volodymyr Zelenskiy to probe former Vice President Joe Biden and his son Hunter, who had business dealings in the country, as well as a debunked conspiracy involving Democrats and the 2016 election, are at the center of the House impeachment probe. The inquiry began after the existence of the first whistleblower's complaint, which focuses on the July 25 phone call between Trump and Zelenskiy, became known. Trump has repeatedly claimed the whistleblower made false claims, though the complaint was corroborated by the call summary his White House released.

Contrary to Trump's assertion, the existence of a second whistleblower was revealed two weeks after the White House released the partial transcript. While Trump decried the idea of the whistleblower providing written answers, he refused to provide Mueller with anything more than that during his years-long probe into Russian interference in the 2016 presidential election. Trump's attorneys warned the president against doing a live interview with Mueller, saying he could be setting up a "perjury trap" for the president.

Others, meanwhile, said the president couldn't possibly perjure himself if he told the truth.

Trump's tweet came amid an increased push in recent days by Trump and his allies to reveal the first whistleblower's identity. On Sunday, Trump suggested the whistleblower "hated Trump," pointing to a media report claiming to have revealed the whistleblower's identity.

"They think they know — they know who it is," Trump told reporters. "You know who it is. You just don't want to report it. CNN knows who it is, but you don't want to report it. ... You’d be doing the public a service if you did."

In response, Zaid said, "The fixation on the whistleblowers is simply because the president (and others) are at a loss on how to address the investigations the underlying disclosure prompted."

Trump's latest tweet came amid a flurry of posts over the past 12 hours, which included him saying Republicans should release their own transcripts from the House hearings, asserting there is "no reason" to interview any witnesses, suggesting there might be nothing wrong with a quid pro quo, and retweeting Mark Levin, a conservative personality who is one of Trump's staunchest defenders, a half-dozen times.