WASHINGTON – President Donald Trump said Monday he is still trying to learn the identity of the whistleblower whose allegations over Ukraine and Joe Biden have triggered an impeachment inquiry – a comment that some critics regarded as a presidential threat against the informer.

"We're trying to find out about a whistleblower," Trump told reporters after an Oval Office swearing-in ceremony for new Labor Secretary Eugene Scalia.

Trump again attacked Rep. Adam Schiff, D-Calif., chairman of the House Intelligence Committee, accusing him of "making up" words about his phone call with the president of Ukraine – but Trump did not again accuse Schiff of "treason" as he did earlier in the day.

After Trump spoke, Andrew Bakaj, the attorney for the unidentified whistleblower, tweeted that his client "is entitled to anonymity. Law and policy support this and the individual is not to be retaliated against. Doing so is a violation of federal law."

Critics said Trump's comments, as well as repeated attacks on the whistleblower's motives, amount to a threat of reprisal against someone seeking to expose government wrongdoing.

"We have a centuries-old bipartisan consensus that those with evidence of wrongdoing should be encouraged to step forward, not intimidated from doing so," said John Kostyack, executive director of the National Whistleblower Center, a nonprofit group that promotes whistleblower protection laws.

Kostyack urged lawmakers from both parties "to affirm that this whistleblower deserves the highest level of protection from retaliation, including the ability to maintain anonymity.”

In his brief Oval Office comments, Trump claimed his accuser misrepresented his July 25 telephone conversation with the president of Ukraine, Volodymyr Zelensky.

Schiff and other lawmakers said a transcript released by the White House backs the whistleblower's story Trump pressured Ukraine to investigate his political rival Biden, who is seeking next year's Democratic nomination for president.

The transcript shows Trump repeatedly mentioning aid to Ukraine in close proximity to his request that Ukraine investigate Biden and his son Hunter Biden, who had business interests in the country.

Trump again called the conversation "perfect," and said he only wanted Ukraine to look into what he called "corruption." He claimed that Ukraine sought to interfere in the 2016 president election in 2016 "against us," but the intelligence community has repeatedly debunked that allegation.

Trump commented as he opened another week of impeachment turmoil by attacking his accusers.

In a series of late-night tweets on Sunday and early-morning Monday, Trump raised the specter of "civil war," accused one key investigator of "treason," said some of his own aides may be "SPYING" on him, and accused the Democrats of trying to "destabilize" the country a year ahead of his 2020 re-election.

"They are lying & cheating like never before in our Country’s history in order to destabilize the United States of America & it’s upcoming 2020 Election," Trump said in one tweet.

On Monday morning, Trump repeated his claims about Schiff in a new tweet and said: "Arrest for Treason."

The angry tweets set the stage for another week of political/legal battles, as House Democrats investigate allegations that Trump tried to coerce the president of Ukraine into investigating Biden, and essentially inject a foreign country into the 2020 presidential campaign.

"We had no choice but to move forward with an impeachment inquiry," Schiff tweeted Sunday. "And our focus will be on the president’s fundamental breach of his oath office."

He added: "Coercing a foreign nation to interfere in our election is never ok, No matter what the president and his defenders say."