A powder found in an office of Senate Majority Leader Bill Frist tested positive yesterday for the lethal poison ricin prompting authorities to impose tight security and carry out street-side decontaminations.

Parts of the Dirksen Senate Office Building, where the powder was found, remained closed early this morning, while a definitive test was conducted on the powder.

Witnesses said that several people apparently had undergone decontamination last night in a van parked outside the building and that as many as 40 people in all would undergo the process before being sent home.

According to officials, the powder was discovered in Room 454, which was described as a mailroom that is part of Frist’s offices.