Squads of fire marshals and police detectives were combing the streets and examining the subway tunnels of Lower Manhattan yesterday afternoon, looking for clues that could lead to the cause of a crippling subway fire that it is estimated will cost as much as $60 million to repair.

It has been widely reported that Sunday’s fire inside the Chambers Street station on the A and C lines, on which restoration of normal service will take up to nine months, was caused by a homeless man who burned pieces of garbage and wood to keep himself warm during the winter chill. Police and fire officials downplayed those reports yesterday as premature and inconclusive. They said the fire, which first consumed electrical lines and then spread into a locked room containing relays, could have been sparked by any number of sources.

The police commissioner, Raymond Kelly, said he would characterize ignition of the smoky fire by a homeless person as “an assumption.”

“You can’t rule it out,” Mr. Kelly said, “but it certainly hasn’t been determined to be a homeless person.”

A spokesman for the Fire Department, which is spearheading the investigation alongside detectives from the Police Department, said fire marshals have already ruled out another theory: that the fire could have been triggered by a mechanical failure in the track’s electrical system or a short-circuit in the wires of the relay room, which was built more than 70 years ago and upgraded incrementally by transit engineers.

“The fire still remains incendiary,” the spokesman, Mike Loughran, said. That means that marshals believe the fire was intentional.

He said marshals had traced the trail of the fire to a pile of “combustibles,” essentially a pile of trash, about 50 feet north of the subway station in the tunnel, under Church Street.

Before sunrise yesterday, plainclothes detectives from the Police Department’s Arson and Explosion squad, who are aiding the Fire Department in their investigation, were questioning homeless people and store owners near the station at Chambers and Church streets for additional information about the fire.

One homeless man, George Johnson, 71, said that about 6:15 a.m., police detectives from Arson and Explosion had questioned him about the fire and said they were looking for another homeless man as a possible suspect. The detectives, he said, described the suspect as disheveled and raucous man wearing a colorful Mexican poncho and pushing a shopping cart. Mr. Johnson said he spends much of his time panhandling for change on the stretch of Chambers between Church and West Broadway.

The description of a homeless man wearing a colorful poncho seems to have come from the manager of the Popeye’s Chicken & Biscuits restaurant on Chambers. The manager, Al Portman, said that about 6:30 p.m. Saturday a homeless man, with two shopping carts and a large bag, came into the fast-food restaurant and began to curse and make a ruckus.

Mr. Portman said the detectives whom he told about the man in the poncho did not appear to be convinced he had anything to do with the fire. Mr. Johnson, too, said police detectives expressed a frustrating concern that conclusively linking an unidentified homeless man to a runaway fire could be a daunting task.

According to statistics recorded on one night last February by the city’s Department of Homeless Services, there were 582 homeless men and women sleeping inside subway platforms or cars in Manhattan and Brooklyn. While some homeless people may fall asleep on stations and trains, a spokesman for the Metropolitan Transportation Authority said that no homeless people live “permanently” inside the subway system.