A D.C. Air National Guard fighter jet based at Andrews Air Force Base accidentally fired about two dozen rounds of ammunition during a nighttime training mission, piercing the roof of a school in southern New Jersey, authorities said yesterday. No one was injured in the incident, which is under military investigation.

Six of the projectiles, at least two inches long and a half-inch in diameter, were found inside Little Egg Harbor Intermediate School yesterday morning. Five were found in the parking lot, said Lt. John O'Brien of the Little Egg Harbor Township police.

Police were called to the school after a custodian heard noises about 9:20 p.m. Wednesday that she likened to people running across the roof, said Allan Bossard, principal of the 970-student school for students in grades 3 to 6. Police found no intruders.

Officers returned yesterday morning after school employees found projectiles inside the building. There was damage to the roof, ceiling tiles and carpeting.

Lt. Col. Roberta Niedt, spokeswoman for the New Jersey Department of Military and Veterans Affairs, said that the gun aboard the jet accidentally discharged. She said investigators were trying to determine whether it was caused by a mechanical malfunction or an error by the pilot of the single-seat F-16 fighter jet.

Col. Jeff Johnson of the 113th Wing of the D.C. Air National Guard said the unit routinely trains at the Warren Grove Gunnery Range, a 2,400-acre area about 3.5 miles from the school in Little Egg Harbor. The plane was flying at about 7,000 feet and was within the range when the rounds were fired, officials said.

"The key is, it was inadvertent," O'Brien said. "It was not directed at the school."

Residents of Little Egg Harbor, a community of 25,000 about 30 miles north of Atlantic City, have long been accustomed to roaring jets flying practice missions. The firing range has been used by the National Guard since the early 1960s.

"We are used to operations out there" at the range, O'Brien said. "It's pretty commonplace to have these aircraft out there. More recently, they have run night sorties, and [they] notify the police department they are out there."

He said no residents called police to report the incident Wednesday night, probably because the Guard flights are so common. He said people had called police after hearing of the incident, but he added, "I don't sense any panic."

O'Brien and Niedt said the incident was the first of its kind in Little Egg Harbor, a town in the Pine Barrens that was named because turtles would lay eggs on its beaches. The town is a bedroom community for Atlantic City and has residents who commute to Philadelphia and New York, officials said.

"The range has been there for 40 years, and there's never been an accident like this," Niedt said.

Johnson said the military took the incident "extremely seriously. The safety of our people in the surrounding community is of foremost concern."

Practice missions at the range have been suspended until the investigation is completed, authorities said.

The 113th Wing of the D.C. Air National Guard has about 1,050 people and includes the 121st Fighter Squadron and the 201st Airlift Squadron, which transports members of Congress and other dignitaries.

Bossard, the Little Egg Harbor school principal, said the custodian, one of four in the one-story building Wednesday night, thought that children might have been on the roof or in the building when she heard sounds above her.

"She went into a room and saw a ceiling tile that looked like it had been poked with a stick, because there was a hole in it," he said.

Bossard expressed thankfulness that no students were at the school when the rounds were discharged. "At least the timing was good," he said. The school had been scheduled to be closed yesterday and today because of a teachers convention. Classes are to resume Monday.

Robert Brown, whose twin daughters attend the school, about a quarter-mile from their home, said he first learned of the incident while watching television yesterday.

"Twenty five rounds? Wow! That can be dangerous," Brown said. "That kind of scares me."

Neighbors said low-flying jets are common. The range is in the Pygmy Forest, named for its five-foot-tall pine trees. Brown said planes sometimes fly so low while approaching the range that he can see the pilots' heads.

"It's kind of like an air show when they're out there," he said.

Elinor Bell, 50, who takes her Great Dane, Axle, on a daily walk to the school, said she is not too worried by the incident, calling it a "freak accident."

"I just think this guy hit the button by accident," she said. "It was scary, but it's like, what are you going to do? Thank God no one got hurt."

Staff writers Maureen Fan, Spencer S. Hsu, Mary Beth Sheridan and Clarence Williams and researcher Bobbye Pratt contributed to this report.

Little Egg Harbor Intermediate School in New Jersey was hit by about 25 20mm training rounds Wednesday night. Little Egg Harbor police returned to the school yesterday after employees found projectiles inside the building.

Col. Brian Webster, left, of the New Jersey Air National Guard, talks to reporters. With him are Lt. Col. Roberta Niedt and Police Chief Mark Siino.