The Democratic National Committee recently reported to the FBI that a security sweep turned up evidence its Washington, D.C. headquarters may have been bugged, two unnamed committee officials told Mother Jones.

According to the report, the DNC ordered two recent sweeps: one after the hack of its employees’ emails and a second after conservative activist James O’Keefe released a hidden camera video suggesting Democrats tried to incite violence at some of Donald Trump’s campaign rallies. The sources told Mother Jones that the second sweep turned up a radio signal near interim DNC Chair Donna Brazile’s office.

“We were told that this was something that could pick up calls from cell phones,” one unnamed DNC official told Mother Jones. “The guys who did the sweep said it was a strong indication.”

No device was ultimately found, according to the report, but the findings from the sweep were sent on to the FBI. The unnamed official would not tell Mother Jones what actions were taken from there, beyond saying “we believe it’s been given by the bureau to another agency with three letters to examine.”

“As a general policy, we don’t talk about such efforts,” the other source told Mother Jones. “You have to take all of this incredibly seriously.”

A spokesperson for the DNC declined to comment on the security sweeps to Mother Jones.

“The DNC is not going to comment on stories about its security,” spokesman Adam Hodge told the site. “In all security matters, we cooperate fully with the appropriate law enforcement agencies and take all necessary steps to protect the committee and the safety and security of our staff.”

The FBI did not respond to Mother Jones’ request for comment.