Washington ? Preliminary tests of a white powder discovered in a Senate office building Monday were positive for the potentially deadly poison ricin, the U.S. Capitol Police chief said.

Two out of three tests indicate ricin, Chief Terrance Gainer said at a late-evening news conference. The third test came out negative, and a fourth, more definitive test was under way, with results expected today.

Sixteen people who were on the floor where the white powder was discovered on mail were being decontaminated and would be allowed to go home, Gainer said.

“At the moment we’re in a wait-and-see position from an analytical point of view in what next steps we may take,” he said. That included what, if any, decontamination of the Dirksen Senate Office Building would be needed.

Senate Majority Leader Bill Frist, R-Tenn., also at the news conference, said no symptoms were reported by those near the powder. “Everybody’s fine” and there is “no cause for alarm,” he said.

“Nobody is sick, we don’t expect anybody to get sick,” said Frist, a surgeon before his election to the Senate.

The powder was discovered about 3 p.m. in a mail room near Frist’s office on the fourth floor of the Dirksen Building, Gainer said. A congressional official had said earlier the powder was found in Frist’s office suite.

Another government official said lab tests were being conducted at Fort Detrick in Maryland, but Gainer refused to give any location.

Authorities do not know whether the substance was found on a letter or a package, the chief said.

Senate Majority Leader Bill Frist, R-Tenn., front left, walks with Capitol Hill Police Chief Terrance Gainer, right, in Washington. Preliminary tests of a white powder discovered Monday in a Senate office building tested initially positive for the potentially deadly poison ricin, Gainer said at a late-evening news conference.

The Homeland Security Department is monitoring the situation, spokesman Brian Roehrkasse said.

Ricin, derived from the castor bean plant, can kill within days. Twice as deadly as cobra venom, ricin is relatively easily made. It may be inhaled, ingested or injected.

Police found traces of ricin in a north London apartment last January and arrested seven men of North African origin in connection with the virulent toxin that has been linked to al-Qaida terrorists and Iraq.

A package containing ricin also was found in October at a post facility serving Greenville-Spartanburg International Airport in South Carolina.

An FBI official said the bureau was awaiting the result of tests at the Fort Detrick laboratory before deciding whether to get more fully involved in the case.

Mail to congressional offices has been irradiated since deadly anthrax was found in letters sent to the offices of Sens. Tom Daschle, D-S.D., and Patrick Leahy, D-Vt., in 2001. No one was arrested in those incidents.

Ricin cannot be destroyed by the irradiation processes, sources familiar with security procedures said.