A rookie firefighter died after the floor of a burning 99-cents store collapsed, plunging him and four colleagues into the basement and burying them under a mountain of debris.

Scores of firefighters were called to the one-story Bronx building around 12:30 p.m. yesterday, and five of them became trapped in the basement after the ground floor gave way while they were searching for victims, Mayor Bloomberg said. The firefighters had to be given oxygen through a special hose before the rubble could be partially cleared and they could be pulled out.

Firefighter Michael Reilly, 25, died of his injuries; the other four firefighters trapped with him also were injured, one of them critically, Mr. Bloomberg said at a somber hospital news conference.

“I ask all New Yorkers to take a moment and say a prayer for the firefighter that we lost,” Mr. Bloomberg said. He recalled meeting Reilly at his graduation ceremony this year, shaking his hand, and presenting him with a plaque.

Reilly, who had been carrying a hose into the burning discount store, was a Marine who served in Iraq and graduated from the fire academy four months ago. He arrived at Bronx-Lebanon Hospital Center yesterday with no pulse or vital signs, and he wasn’t breathing on his own, a doctor said. He was pronounced dead about 3 p.m.

Fire Lieutenant Howard Carpluk, a 20-year veteran, was in very critical condition at Montefiore Medical Center.

Battalion Chief Thomas Auer, 47, a 23-year veteran, Lieutenant John Grasso, 45, a 21-year veteran, and firefighter Wayne Walters, 36, a four-year veteran, were in serious but stable condition, Mr. Bloomberg said.

Several injured firefighters were seen coming out of the gutted store, which sold discounted toiletries, kitchen supplies and knickknacks. About 20 were treated for smoke inhalation.

Employees at the store called 911 when smoke started billowing from behind a refrigerator and they couldn’t stop it with a fire extinguisher, the mayor said. The glass doors were blown out of the building, which had a clean structural history, a buildings department spokeswoman said.

The fire was under control by 4:45 p.m. No cause had been identified, but the fire was not deemed suspicious.

The mayor met with the parents and brother of the dead firefighter, who joined the U.S. Marine Corps in 2000 and was a sergeant in the Marine Reserves.

The family members told the mayor their loved one had always wanted to be a firefighter.

“I think they can be very proud that their son did what he wanted to do,” Mr. Bloomberg said.

Last year, two firefighters died when they jumped from the fourth floor of a blazing building in the Bronx.

They had become trapped on January 23, 2005, while searching for people inside. Four others were critically injured as they jumped from the building, where illegally built partitions had blocked access to a fire escape.

Reilly, of Sleepy Hollow, N.Y., is the 1,133rd firefighter to be killed while working for the Fire Department of New York in its 142-year history.