The FBI has asked to interview the CIA whistleblower whose complaint touched off the Ukraine impeachment investigation, a source directly familiar with the matter told NBC News.

The whistleblower has not yet agreed to an interview, the source said.

A law enforcement source said the FBI is not investigating the whistleblower — the bureau wants to talk to him.

The outreach to the whistleblower was approved by the FBI’s Washington field office, and neither Attorney General William Barr nor those in his orbit at the Justice Department were made aware until media reports surfaced on Wednesday, according to another source familiar with the matter.

The FBI request was first reported by Yahoo News, which said that some FBI officials were disturbed that the Justice Department declined to investigate the whistleblower's complaint after a criminal referral was sent over from the inspector general of the Intelligence Community.

Spokespeople for the FBI and the Justice Department did not immediately respond to requests for comment.

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Justice Department officials said they examined the criminal referral based on the whistleblower's complaint, and decided that there should be no investigation. They said they only examine the question of whether a campaign finance crime occurred, and they have never explained why they did not consider questions of bribery, extortion or other possible crimes.

The whistleblower was not on the July 25 call between President Donald Trump and the Ukrainian president, and is not considered a first-hand witness to any of the key moments in the Ukraine saga. The whistleblower aggregated the concerns passed on by other colleagues on the National Security Council, and forwarded them in a written complaint to the inspector general for the intelligence community.

Because the whistleblower is not a first-hand witness, congressional Democrats have decided they do not need the person's testimony and the ongoing impeachment hearings. Republicans, on the other hand, have urged that the whistleblower be brought in to testify, in what critics see as a bid to expose the person's identity.