PALM DESERT – A suspicious letter addressed to comedian Bill Maher which contained a white powder substance was received at the McCallum Theatre in Palm Desert Friday, prompting an evacuation of the venue.

The Riverside County Sheriff's Department was notified of the letter at the theater at 73000 Fred Waring Dr. around 11:40 a.m., sheriff's Deputy Herlinda Valenzuela said.

Riverside County sheriff's Cpl. Matt Spain told mydesert.com, The Desert Sun's Web site, that it was addressed to Maher, the host of HBO's “Real Time with Bill Maher,” who is scheduled to appear at the theater Saturday night.

“It contained specific threats made against Bill Maher,” Spain told mydesert.com, declining to go into specifics.

A deputy sent to investigate the discovery was not able to identify the substance inside the letter, but did determine that it was not a controlled substance, Valenzuela said.

County firefighters along with personnel from the county's Environmental Health Department later arrived at the scene to assist Palm Desert police with evacuating employees inside the venue as a precaution, she said.

Four people who came into contact with the powder inside the theater were decontaminated, Valenzuela told the Sun.

The building was shut down for more than five hours and tonight's concert featuring singer-guitarist Boz Scaggs was canceled, according to the Sun.

A Hazmat team collected and secured the letter and the substance was forwarded to the FBI for testing and disposition, Valenzuela said.

FBI spokeswoman Laura Eimiller told mydesert.com that the powdery substance was not harmful.

The FBI Joint Terrorism Task Force was continuing to investigate the incident, mydesert.com reported.