A bomb threat was sent to the headquarters of National Public Radio on Oct. 25, five days after the firing of controversial political pundit Juan Williams, the FBI said.

According to The Washington Post, sources at NPR said the news organization received the threat via U.S. mail and then forwarded the messages to local authorities and the FBI. The organization also asked its employees to be more vigilant in the wake of the threat.

“We’re taking extra precautions today,” NPR spokeswoman Anna Christopher told the Post on Oct. 26. “We’re being more aware of who’s entering the building.” No explosives were found.

Williams was fired after an appearance on Fox News’ “The O’Reilly Factor,” during which he expressed anxiety about flying on planes with people dressed in “Muslim garb.” Later in that interview, he spoke out against profiling, but NPR said the comment was part of a series of pre-existing disagreements the organization had with Williams over his appearances and comments on Fox News. Following his firing, Williams signed a multi-million dollar deal with Fox News.