Less than two weeks after a relayroom fire in a subway station at Chambers Street disrupted service for more than a half-million subway riders, the New York Fire Department terminated its investigation into the blaze yesterday, without ascertaining its cause.

Initially, the Fire Department classified the blaze as “incendiary,” or resulting from a fire that was set, and the Metropolitan Transportation Authority assigned blame for the fire to an unidentified homeless person, citing reports of debris burning in a shopping cart and testimony by witnesses about a mysterious vagrant with a Mexican style poncho lurking around the subway station near the time of the fire.

Police eventually located the poncho-wearer but did not take him into custody, and the homeless-igniter theory was downplayed as an “assumption” by the police commissioner, Raymond Kelly. Yesterday, the fire’s official status was changed from “incendiary” to “not ascertained,” meaning the cause could not be determined.

“This case is basically closed,” a Fire Department official said.

“Fire marshals in conjunction with the NYPD Arson and Explosion squad and Transit Bureau detectives conducted a thorough investigation into the circumstances surrounding the fire in the subway on January 23, 2005, and there was no conclusive information to arrive at a definitive cause for this fire,” the department said in a statement.

A department official, who asked that his name be withheld, said that while there was no limited window of opportunity for collecting evidence and testimony, the department would not continue to try to find the cause of the fire, which shut down the C line, curtailed service on the A, caused some trains to be rerouted, interrupted the daily commute of 580,000 riders, and stands to cost the MTA as much as $60 million.

“We’ve exhausted all options,” the fire official said, adding that the investigation could be reopened if a witness came forth with new evidence.

Given the department’s ruling, a senior policy analyst of the Coalition for the Homeless, Patrick Markee, said: “It’s just outrageous that they were out there accusing a homeless person of committing this act last week when they clearly had no evidence to back up that accusation.”

Fire Department sources said the investigation probably suffered from a lack of manpower, owing to two other blazes that competed for department resources January 23.

One of the fires ravaged an apartment building in the Bronx, where two firemen were killed attempting a 50-foot jump to escape the flames. Another fireman’s life was claimed in a Brooklyn apartment-building fire.

A department official said fire marshals were called off other investigations that Sunday to look into the blazes that killed their fellow firefighters.

Some off-duty marshals were pressed into service, he said, although not all of the department’s marshals – 100 to 120, by his estimate – were sent to investigate the three major fires on January 23.

Some of the marshals, he said, were sent to accompany the bodies of the dead firemen to the morgue and remain with them until autopsies were conducted – a practice the spokesman said is department policy.

The Fire Department spokesman said that a “not ascertained” ruling “doesn’t happen often, but it’s not uncommon.” In 2003, of the estimated 4,200 fires investigated by fire marshals, 40 – just under 1% – were labeled “not ascertained.”

An MTA spokesman, Thomas Kelly, said that despite the inconclusive Fire Department ruling, the authority plans no independent investigation of the fire at this time, and he expressed no disappointment that the cause of the fire could not be determined.

“We’re sure the Fire Department did everything they could,” Mr. Kelly said.

It is possible, however, that another investigation could be opened by the MTA’s insurer, the First Mutual Transportation Assurance Company, according to an insurance-industry authority.

The director of public affairs for the state Insurance Department, Michael Barry, said it is common industry practice for insurers to conduct their own investigations into claims in the absence of a definitive ruling from law-enforcement bodies.

“It’s safe to say that now that the Fire Department has closed the investigation … standard practice would be” for the insurance company to dispatch its own investigator and compile its own report, Mr. Barry said.

Mr. Kelly said, however, that the cause of the fire would not affect the MTA’s insurance payout. The MTA said late last week that the payout would be modest, because the equipment that was destroyed, which was more than 70 years old, was insured at its current value.

A spokesman, Daniel Prince, for Willis Coroon, the firm that manages First Mutual – a state entity created in 1998 – declined to comment, citing concerns for his client’s privacy.