The fire alarm which caused the evacuation of Thayer Hall Friday night was set off by the smoldering ashes of plastic objects in a paper bag, Sgt. James McCarthy of the Harvard Police said yesterday.

McCarthy said that when fire-fighters arrived, they found a smoking plastic bag in the room but have no idea how the fire started.

"A bag doesn't spontaneously combust," McCarthy said, adding that the students "did something really dangerous."

Although the ashes were contained within the bag and the room sustained no damage, McCarthy said, the air was so thick with smoke that firefighters had to search the room by touch.

George A. Economou '00, a fifth-floor Thayer resident, said the building's hallways were full of smoke from the third floor up to the top.

"The dorm was filled with smoke," Economou said.

"Everyone was running down the stairs saying, 'Get out! Get out!,'" said fourth-floor resident Grace Liu '00.

"There was thick smoke throughout the building."

Frances Wall '00, who lives on the second floor, said she could see gray smoke coming down from the third floor as she evacuated.

"The odor was intense, like paper or wood burning," Wall said. "The smell had really permeated the building."

Two first-year students, who requested anonymity, said the smoke smelled like burning marijuana.

"It smelled a lot like cigarette smoke, or it could have smelled like pot," said a third-floor resident.

"It was obvious that they were smoking pot up there," a second-floor resident said. "The smell was really heavy on the second floor."

"Usually, we could only smell [pot] when we went through the third floor," the student added.

But another Thayer resident said although the dormitory often smells of pot on weekends, by the time the dorm was evacuated on Friday night, "it just stunk like burnt paper," he said. "It was a lot bigger fire than just a joint."