NPR received a bomb threat Monday, five days after its decision to fire news analyst Juan Williams sparked a hugely negative reaction.

Sources at the news organization said the threat was received via U.S. mail and was immediately turned over to local police and the FBI. The organization did not publicly disclose the threat or release details, on the advice of law enforcement officials.

The letter didn't reference the Williams firing specifically, but people at NPR, who spoke about it on the condition of anonymity, said the timing and tone suggested it was sent after Williams's widely publicized termination.

Williams was dismissed as an NPR analyst late Wednesday after he appeared on Fox News Channel's "O'Reilly Factor" and expressed his personal anxiety about flying on planes with people in "Muslim garb." Later in the same interview, however, he spoke out against profiling people on racial or religious grounds.

NPR officials said the remark was the last straw in a long series of disputes with Williams over his comments and appearances on Fox News.

NPR warned its employees Monday about a general "security threat" in a staff memo, but did not spell out the nature of the threat. The radio and digital news organization is headquartered in its own building in downtown Washington.

"We're taking extra precautions today," NPR spokeswoman Anna Christopher said Tuesday, but she did not say what prompted the precautions. "We're being more aware of who's entering the building" and generally being more vigilant. She said the organization's security staff has been making extra checks of the building's garage.

NPR has received thousands of e-mails and phone calls about the Williams episode, most of them critical of its decision. In addition, NPR President Vivian Schiller has received threatening phone calls to her home, some of which were referred to law enforcement. Schiller was not available for comment Tuesday.

Fox News has been especially critical of NPR's action. The cable news network has devoted hours of airtime to the subject, with many of its hosts and analysts calling on Congress to eliminate federal funding for public broadcasting.

On Monday, a producer for the O'Reilly show staged an "ambush" interview of Schiller, peppering her with questions as she emerged from a car on a Washington street. Schiller declined to respond.

Fox's coverage of the Williams firing has been so intense that it was satirized on Comedy Central's "Daily Show" on Monday.