Authorities are investigating a package labeled "anthrax" and containing white powder that was opened in a mail handling room at an Internal Revenue Service mailroom in Missouri Wednesday morning.

Two IRS employees were exposed to the package, whose return address was a prison, the Kansas City Star reported Wednesday.

Hazardous materials crews, police, an ambulance, the FBI and U.S Treasury officials arrived at the scene to investigate.

Emergency workers said the mail room has a separate ventilation system from the rest of the building.