WASHINGTON (Reuters) - U.S. authorities have taken security measures to protect a U.S. intelligence official who filed a whistleblower complaint that sparked an impeachment inquiry against President Donald Trump, a source familiar with the case told Reuters.

Lawyers for the whistleblower expressed concern in a Sept. 28 letter to the acting director of National Intelligence, Joseph Maguire, that their client’s safety could be at risk after Trump suggested he was a spy who had committed treason.

They said certain individuals had offered a $50,000 “bounty” for any information on the whistleblower’s identity after the official complained to a government watchdog that Trump had pressured Ukrainian President Volodymyr Zelenskiy in a July 25 phone call to investigate his political rival, Joe Biden.

The U.S. government source familiar with the whistleblower’s case said federal authorities were taking precautions to protect the official, but declined to say what measures had been put in place. The source also declined to say whether any specific threats had been made against the whistleblower.

A lawyer for the whistleblower declined to comment on his client’s security.