KANSAS CITY, Mo. -- A letter containing a white powder and a note mentioning anthrax forced federal authorities Wednesday to shut down the mailroom of the Kansas City headquarters of the Internal Revenue Service.

The letter was sealed and sent to a lab in Jefferson City for further testing, said FBI spokesman Bob Herndon.

"We do not think this is going to be anthrax or any other biological agent, but we have to treat this to the n-th degree," Herndon said, adding that a field test found the substance likely to be talcum powder.

A mailroom employee who handled the letter went through decontamination procedures, Herndon said.

He said the mailroom was being sealed off until the test results came back late Wednesday or early today. The facility has a different ventilation system than the rest of the IRS headquarters.

Herndon refused to comment on earlier reports that the letter had a return address of a state prison, other to say investigators were following a "viable lead" to the letter's sender.