Multiple air traffic controllers in New York heard a chilling threat Monday in audio obtained exclusively by CBS News: “We are flying a plane into the Capitol on Wednesday. Soleimani will be avenged.”

The threat refers to Qassem Soleimani, the Iranian general killed last year in a U.S. drone strike ordered by President Trump. It was made on the one-year anniversary of Soleimani’s death, for which Iranian officials have long vowed revenge.

It’s unclear who sent the threat. While the government does not believe the warning of an attack is credible, it is being investigated as a breach of aviation frequencies, CBS News has learned.

Sources told CBS News that the Pentagon and other agencies were briefed Tuesday about the digitized voice recording.