Lawyers for the intelligence official who filed a whistleblower complaint against President Donald Trump released a letter Sunday in which they said they had "serious concerns" for their client's safety.

Compass Rose Legal Group said recent events had raised concerns that their client's identity might be disclosed publicly, which could put the person at risk of retribution.

In particular, the lawyers noted closed-door comments made by Trump on Thursday in which he characterized the intelligence official as a "spy" who perhaps committed treason, a crime punishable by death.

The lawyers also said people had issued a "$50,000 bounty" for information regarding their client's identity.

Trump on Sunday demanded to meet with the whistleblower and doubled down on claims that those involved in the complaint might have committed treason.

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Lawyers for the intelligence official who filed a whistleblower complaint against President Donald Trump that triggered a formal impeachment inquiry have raised "serious concerns" for their client's safety.

In a letter first published by CBS News' "60 Minutes," lawyers from Compass Rose Legal Group said events from the past week had "heightened our concerns that our client's identity will be disclosed publicly and that, as a result, our client will be put in harm's way."

In particular, the lawyers noted comments made by Trump on Thursday in which he characterized the intelligence official as a "spy" who perhaps committed treason, which is punishable by death.

"I want to know who's the person that gave the whistleblower, who's the person that gave the whistleblower the information, because that's close to a spy," Trump said during a private event in New York on Thursday, which the Los Angeles Times obtained audio of. "You know what we used to do in the old days when we were smart? Right? With spies and treason, right? We used to handle them a little differently than we do now."

The lawyers called on leading Democrats and Republicans to speak out for whistleblower protections and stress that retaliation against the unnamed official "whether direct or implied" would not be tolerated.

They also said "certain individuals" had issued a "$50,000 bounty" for information regarding their client's identity.

"Unfortunately, we expect this situation to worsen, and to become even more dangerous for our client and any other whistleblowers, as Congress seeks to investigate this matter," the letter said.

CBS News previously reported that the whistleblower was already under federal protection. But Mark S. Zaid, a lawyer from Compass Rose Legal, wrote on Twitter that CBS "completely misinterpreted contents of our letter" and said the legal team had not yet reached an agreement with Congress regarding contact with its client.

Read more: Trump is facing the biggest firestorm of his presidency because his own staffers blew the whistle on him

Trump on Sunday demanded to meet with the whistleblower and doubled down on claims that those involved in the complaint might have committed treason.

"Like every American, I deserve to meet my accuser, especially when this accuser, the so-called 'Whistleblower,' represented a perfect conversation with a foreign leader in a totally inaccurate and fraudulent way," Trump wrote in a series of tweets Sunday night.

The whistleblower's complaint had centered on Trump's communications with Ukraine's president and alleged Trump might have been using the power of the presidency for personal gain.

The US House Intelligence Committee's chairman, Rep. Adam Schiff of California, told ABC's "This Week" on Sunday that he expected the whistleblower to testify "very soon" once security measures were in place to protect the person's identity.

House Speaker Nancy Pelosi announced last week that the House would launch a formal impeachment inquiry, in light of a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky that is a focal point of the whistleblower complaint.

The intelligence community inspector general, Michael Atkinson, deemed the August 12 complaint "credible" and of "urgent concern" and passed the complaint on to the acting director of national intelligence, Joseph Maguire.

Maguire testified before the House Intelligence Committee on Thursday and said that both the whistleblower and the inspector general "acted in good faith throughout" and "have done everything by the book and followed the law."