Early tests show deadly ricin in Senate mailroom.

Preliminary tests on a white, powdery substance found in the mailroom of Senate Majority Leader Bill Frist indicate the presence of the deadly substance ricin, a Homeland Security official said Monday.

A U.S. Capitol police spokeswoman said the department is investigating the matter but would not comment on the substance found, saying only that preliminary tests were positive for a “hazardous substance.”

The substance is to be tested further at the Army research laboratory at Fort Detrick, Maryland.

The Homeland Security official, who spoke on condition of anonymity, said those results could be back as soon as Tuesday.

Authorities said people should stay clear of the south side of the fourth floor of the Dirksen Office building.

Ricin is a natural, highly toxic compound that comes from castor beans, used to make castor oil. It can be inhaled, ingested or injected.