A seven-alarm fire ripped through an abandoned skyscraper next to Ground Zero in Lower Manhattan Saturday, killing two firefighters who were responding to the blaze.

Officers at the scene were preventing nearby residents from returning to their homes, telling them that authorities were concerned the former Deutsche Bank office building, vacant since the 2001 terrorist attacks turned it into a toxic nightmare, could fall. Mayor Michael Bloomberg said that fear turned out to be unfounded.

The plume of gray smoke that trailed above the site of the World Trade Center raised concerns that toxic substances in the building could be spreading.

• PHOTO ESSAY: Fire Erupts at Skyscraper at Ground Zero

Bloomberg sought to reassure residents that the chemicals in the building likely did not present a significant health risk, saying air-quality tests so far showed no danger.

"Having said that, we are extremely careful. We don't want to prejudge anything," the mayor added. Tests were to continue overnight, he said.

One of the firefighters killed was identified as Joseph Graffagnino, 34, of Brooklyn. He was a member of Ladder 5, which lost 11 members on Sept. 11, 2001.

"Today's events really are another cruel blow to our city and to our fire department," Bloomberg said. He said the fire had "expanded our loss."

Also killed was Robert Beddia, 53, of Staten Island. Bloomberg said both firefighters had become trapped, inhaled a great deal of smoke and gone into cardiac arrest.

Five or six other firefighters were taken to a hospital but were expected to be released, Bloomberg said. No civilians were hurt.

Construction crews had already dismantled 14 of the building's 40 stories — reaching the 26th floor on Tuesday. Some firefighters used stairs to reach the burning upper floors of the building, just steps from where 343 firefighters lost their lives in the 2001 terror attacks.

The cause of the fire was not immediately known. Smoke pouring from the burning building was visible from midtown Manhattan and the New Jersey side of the Hudson River. Fire officials declared the blaze under control late Saturday.

The acrid smell of smoke, which hung over the neighborhood for days after Sept. 11, returned to lower Manhattan along with the wail of emergency vehicles. More than five dozen fire vehicles, with more than 270 firefighters, responded to the blaze as pieces of burning debris fell from the building to the streets.

Residents said they weren't allowed home even to rescue their pets.

"We heard this crackling," said Elizabeth Hughes, who saw the fire start from her rooftop deck across from the tower. "And then a huge fire that went up three floors fast. It was massive. ... Oh my God! I can't even go in and get my cats."

By late Saturday evening, nearby residents who had been evacuated were told they could return.

The 1.4-million square foot office tower was contaminated with toxic dust and debris after the World Trade Center's south tower collapsed into it. Bloomberg said the chemicals in the building did not present a significant health risk.

Efforts to dismantle it were halted by a labor dispute last year, along with the ongoing search for the remains of attack victims.

City officials announced in June they had completed recovery efforts at the structure. More than 700 human remains were found at the site.

Errol Cockfield, a spokesman for the Empire State Development Corp., which is overseeing redevelopment at Ground Zero, said authorities were investigating whether the smoke at the scene could pose any environmental danger.