A Prince George County firefighter was killed last night in the second of two closely spaced explosions, apparently caused by natural gas, that tore a brick wall from a three-story Glenn Dale apartment building.

The blasts were attributed to ignition of gas that accumulated after children swinging on gas lines in a laundry room broke open a line that fed a clothes dryer.

Firefighters, summoned to the building at 9923 Good Luck Rd. about 5 p.m. by the report of the first blast, found flames leaping from second-floor windows. As they pulled hoses toward the building, the second blast ripped the building open.

"The whole front blew out," said Capt. William Goodwin. "Everything came apart."

Firefighter Thomas L. Graves Jr., 31, was buried by debris. He was the county's first career firefighter killed in the line of duty since the career service was inaugurated in 1966.

In what another fire official called "a miracle," no injuries were reported to any of the residents of the 12-unit building.

Damage to the building and its contents was estimated at about $250,000, and 36 families were displaced last night after gas was turned off at the damaged building and two others in the same complex.

When what apparently was the first of the explosions occurred, Laura Devore was baby-sitting in the building at 9221 Good Luck Rd., and heard shouts coming from outside.

On going to look, she saw smoke billowing from the neighboring building and patio doors dangling from a wall.

"A woman was sitting in a living room, and all of a sudden her sliding doors blew out," Devore said. "she grabbed her baby and another child and got out just in time."

Expressing amazement that none of the many children living in the complex was injured, Devore said. "It was a nice day, and I guess they were all out playing."

The building at 9923 was almost empty when the blast occurred, according to resident manager Carol Bracaglia. "People who should normally have been home just weren't there," she said. "It was as if someone was watching out for them."

As many as 40 firefighters were dispatched to the scene, and the blaze was brought under control within 10 minutes after the second explosion. A team of "probably half a dozen" investigators, aided by a construction crane, remained at the site last night to obtain details about the cause of the blast and fire, officials said.

Susan Butz, a spokeswoman for the Washington Gas Light Co., said several children had been swinging on gas lines that ran near the ceiling of the laundry room.

One of the lines, an inch in diameter, was attached to the dryer. Butz said the children broke it and "pulled it loose." Then, she said, with gas blowing into the laundry room, the children left.

Exactly what ignited the gas was not immediately known last night. "It could have been a number of things," Butz said.

After being struck by the debris, Graves, a member of the Glenn Dale fire company, was taken to Doctor's Hospital of Prince George's County. He died 45 minutes after arrival.

A career firefighter for seven years, Graves lived in New Carrollton and is survived by his wife and a 16-month-old son.

Calling Graves' death ironic, resident manager Bracaglia said the firefighter was personally acquainted with the apartment complex. When a smoke-detector system was installed there two years ago, she said, Graves drew up accompanying plans that showed utility cutoff values.

He had revisited the complex when questions arose about the system and the plans, and "we kind of knew him," she said.

Last night, she said, when the emergency plans were consulted, authorities noted that they bore Graves' initials.

In 1973 a county VFW post designated Graves as its professional firefighter of the year in recognition of his rescue of a child from a burning house.

Although Graves was the first of the 425 members of the county's career fire service to be killed in the line of duty, several of the 2,500 volunteers in the county have been killed while answering calls in recent years.

On June 15, 1978, James Michael O'Conner, a volunteer fireman, was killed when the fire truck on which he was riding collided with a tractor-trailer in Upper Marlboro. He was the 11th county fireman to die on duty. The volunteer system goes back to the turn of the century.

Graves, according to one fire official, was tragically in the wrong place at the wrong time when the building exploded.

"From what I hear," said Capt. James Mundy, "if he'd been 25 feet either way [from where he was] he'd have been okay."