A toxic substance in a stairwell of a government building in the Foggy Bottom area of Northwest Washington sent three people to the hospital and forced an hourlong evacuation of the premises yesterday morning, authorities said.

The building, in the 2400 block of E Street NW and across the street from the State Department, is leased to the Central Intelligence Agency, said Navy Lt. Joy Hopkins, a spokeswoman for the Navy Bureau of Medicine and Surgery, which is based there.

The incident occurred about 9:40 a.m. District fire officials said they received a report of an odor in the building, and the first units on the scene found several people complaining of "respiratory problems."

The department's hazardous materials unit was summoned, but fire officials were unable to determine what the susbstance was because it had all but dissipated. Capt. Ted Holmes, a fire department spokesman, said the substance could have been tear gas or mace.

Three people, all in their twenties, were taken to George Washington University Hospital, where a spokeswoman said they were "treated for exposure to toxic gas" and released.

The building is in a restricted, secure area, and firefighters were accompanied by about a dozen security guards while they were at the site.

The secrecy surrounding the site made it difficult to assess the situation yesterday, and it is unclear how many people were evacuated, how large the building is or which agency is investigating the incident.

A spokeswoman for the naval bureau said the building houses offices of the CIA, but the agency itself would neither confirm nor deny this.

A CIA spokesman referred questions to the General Services Admininstration and declined to answer questions about the incident. The CIA, as a matter of policy, does not confirm or deny employment of specific individuals and it could not be learned whether those treated were connected to the CIA.

A spokesman for the General Services Administration said the building's management is under the direction of the GSA's Northwest field office. Officials there could not be reached late yesterday.