A U.S. Capitol Police sergeant was accidentally shot and killed yesterday by another police officer during a training exercise in a vacant Capitol Hill building, according to police.

Sgt. Christopher S. Eney, 37, was shot in the lower back while he and other Capitol Police officers were nearing the end of a tactical training exercise designed to help them deal with hostage situations.

Capitol Police Chief James M. Powell said that Eney and Officer John Gott, 32, identified as the man from whose gun the fatal shot was fired, were members of the Capitol force's SWAT team. Powell said that the two men were close friends, as were their wives.

Powell said the two officers and other members of the team had concluded the training exercise in a building near the Capitol grounds. The drill had been conducted with blanks loaded in 9 mm automatic pistols, but members of the team had reloaded their weapons with live ammunition after its conclusion, Powell said.

For an unexplained reason, Eney and Gott -- and perhaps other officers -- decided at that point to go through one more exercise, the chief said. It was then that the fatal shot was fired, Powell said.

"Capitol Police were conducting tactical training exercises with unloaded weapons," Harry B. Grevey, deputy chief of the Capitol Police said earlier last night in a statement released several hours after the 3 p.m. shooting. "At some point near the conclusion of the exercise, one of the officers apparently loaded his weapon, and accidental discharge occurred, striking . . . Eney in the lower portion of his back."

Eney, a 12-year veteran of the force and the father of two small daughters, was rushed to the Washington Hospital Center's Medstar Unit, where he died at 4:05 p.m. of massive abdominal injuries, according to a hospital spokesman.

Gott also is a 12-year Capitol police veteran. He was placed on administrative leave pending further investigation by the District's homicide squad in conjunction with U.S. Capitol Police.

The shooting took place on the fourth floor of a vacant building at 128 C St. NE, a six-story brown brick facility that police said had been used in the past for training exercises.

The building, which neighbors said has been empty for a year, is in a residential area across the street from the rear of the Hart Senate Office Building.

Grevey said that training exercises are often conducted by the Capitol Police and that yesterday's was "primarily a hostage-type exercise."

Grevey and other police officers declined to describe the exact circumstances of the shooting or the positions the two men were in when it occurred.

Eney lived in the 2100 block of Beechwood Road, a quiet community of trim lawns and flowerbeds in Lewisdale, a subdivision of West Hyattsville. Neighbors there, some of whom heard of the shooting on the evening television news, expressed affection for the slain officer and shock at his death.

"I'm not dismayed, I'm beside myself," said white-haired J. Morgan Matthews, as he removed his glasses to dry his eyes. "He was a devoted family man. He loved those two daughters of his."

"It's just so hard to believe this now," said Roman Dingler, a neighbor and 35-year resident of the block.

Eney and his wife Vivian had lived in the subdivision for eight years, according to neighbors. Just two weeks ago, they said, he was outside painting the brown trim on the family's one-story brick house. The house is on a corner lot, shaded by a huge tree, with a playhouse in the back yard for the girls.

Eney attended the Church of the Atonement, according to neighbors.

Donald Ritchie, associate historian in the Senate Historical Office, told the Associated Press there is no record of a Capitol police officer being killed in the line of duty since the force began with one man in 1801.

Eney, the AP said, was assigned to the House Security Branch as a plainclothes officer who provided security at the doors and in the third-floor galleries of the House of Representatives, where the public is allowed to watch House proceedings.

Police yesterday gave no indication that they considered the shooting anything but an accident.

Following its investigation of the shooting, it will be up to the U.S. attorney's office to decide whether the case should be presented to the grand jury for action.

But a high D.C. police official familiar with the shooting said yesterday it seemed unlikely that the case would go to the grand jury.