The Democratic National Committee recently told the FBI that it collected evidence last month suggesting that its Washington offices had been bugged, two party officials told Mother Jones magazine.

"We were told that this was something that could pick up calls from cellphones," one official said about a radio signal found near the chairman's office that indicated that a listening device might be located outside the office. "The guys who did the sweep said it was a strong indication."

No device was recovered, however. No possible culprits were identified, according to the report.

Both DNC officials asked not to be identified.

The signal was found in a second sweep of the DNC's offices in late October that had been ordered by Interim Chair Donna Brazile.

She moved after undercover reports released by James O'Keefe and Project Veritas showed former DNC operatives talking about planting protesters to incite violence at Donald Trump rallies and bringing in citizens to commit widespread voter fraud.

The DNC officials told Mother Jones that party officials were concerned that Republican supporters might have infiltrated Democratic offices, where volunteers work phone banks and perform other election duties.

The committee's offices were first swept for surveillance devices in September, the officials said, after a hack of its email system and those of other Democratic Party operations. U.S. intelligence agency's blamed that hack on Russia.

"This examination found nothing unusual," Mother Jones reports.

However, NBC News reported that a DNC official said that there was a "frequency found during a sweep of the building at two different occasions" without being more specific.

The committee provided technical details of its findings from the second sweep to the FBI, the officials told Mother Jones.

"We believe it's been given by the bureau to another agency with three letters to examine," the first DNC official told Mother Jones. "We're not supposed to talk about it."

One Democratic consultant who has done work for the committee, who asked not to be identified, said he was recently informed about the suspected bugging.

The officials would not say what countermeasures have since been taken.

"As a general policy, we don't talk about such efforts," the second official told Mother Jones. "You have to take all of this incredibly seriously."

The first DNC source noted, however, "We are the oldest political party in this country, and we are under constant attack from Russia and/or maybe others."

Adam Hodge, a committee spokesman told Mother Jones: "The DNC is not going to comment on stories about its security.

"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 a request for comment, Mother Jones reports.