The death of a 20-year veteran firefighter early yesterday was the second to result from a three-alarm fire in a 99-cent store in the south Bronx on Sunday, the mayor and fire commissioner said.

Howard Carpluk, 43, a lieutenant assigned to Engine Company 42 in the Bronx, died at 10:42 a.m. in the Montefiore Medical Center surrounded by his family and co-workers. His wife, Debra, and two children, Paige, 10, and Bradley, 14, survive him. The death came one day after a probationary firefighter six months out of the academy, Michael Reilly, 25, died fighting the fire. Reilly was assigned to Engine Company 75.

“For more than 20 years, Lieutenant Carpluk devoted his life to saving others,” Mayor Bloomberg said. “When I met the men of the Engine 42 this morning, they told me how the lieutenant faced each and every challenge before him bravely and unflinchingly. Yesterday was no different.”

A group of five firefighters entered the building to look for survivors not long after 1 p.m. on Sunday, but the ground floor collapsed, trapping them in the basement. Carpluk and Reilly were together carrying a water hose when the floor gave way. A special oxygen hose was inserted into a hole in the rubble to help the five men breathe while other firefighters scrambled to remove enough debris to get them out.

Of the 138 firefighters who responded to the fire, 23 were injured, four of them seriously. Carpluk was listed in extremely critical condition Sunday night, and died yesterday morning. A battalion chief, Thomas Auer, was hurt as well, as was another lieutenant, John Grasso, 45, and Wayne Walters, 30. All three were yesterday listed in serious but stable condition at Jacobi Medical Center.

Sunday was the worst day for the Fire Department since January 2005, when two firefighters fell to their deaths trying to escape from a residential building on fire in the Bronx, and another died looking for survivors in a blazing home in Brooklyn. Firefighters are now equipped with safety ropes to help them escape out of windows of buildings on fire.

“In less than 24 hours, we have lost two courageous men, a young probie at the start of his career and an accomplished veteran who dedicated 20 years of service to New York City,” the fire commissioner, Nicholas Scoppetta, said. “It is a heartbreaking loss for the Fire Department whose members know all too well of the great sacrifice that firefighters are called upon to make in order to protect us all.”

Fire marshals were investigating the cause of the fire at the one-story store at 1575 Walton Ave., but their preliminary analysis showed it wasn’t suspicious. The mayor said the fire appeared to have started behind a refrigerator, and store employees were unable to extinguish it. The fire was under control by 4:43 p.m.

Fire officials said Carpluk had been given two awards for bravery during his career, one for rescuing two unconscious men in the bedroom of an apartment that was on fire in the Bronx in 1988.

Reilly, a native of Sleepy Hollow, N.Y.,was a veteran of the United States Marine Corp, which brought him to Iraq. He left the marines and joined the fire department on April 11th, though he was still in a sergeant in the Marine Reserves. He graduated from the fire academy on July 6. His mother, Monica, father, Michael, brother, Kevin, and sister, Erin, survive him.