At dusk on Thursday, less than 24 hours after the worst mass murder in this city's history, acting Police Chief Ralph Mendoza and two other officers stood transfixed in front of a TV screen.

At least two people had videotaped the attack in Wedgewood Baptist Church, where Larry Gene Ashbrook shot 14 adults and teenagers, seven of them fatally, before killing himself. A day later, as Mendoza peered at the screen, studying Ashbrook's face and listening intently, trying to count the gunshots, he could feel his professional detachment giving way to rage.

"To watch him just do what he did, and not seem to worry about it, not seem to be panicked about it--it's extremely angering," Mendoza said today.

More than 150 people, mostly teenagers, had gathered in the sprawling, red-brick church Wednesday night for a concert by a Christian rock band. Just before 7 p.m., Ashbrook, wearing jeans, a cowboy shirt, a black windbreaker, white tennis shoes and a baseball cap, stepped into the sanctuary. The unemployed, 47-year-old loner had already shot three people in the church lobby.

As shots rang out in the sanctuary, the two cameras turned from the band to focus on Ashbrook. In his right hand was a semiautomatic Ruger P85 handgun.

What astonished Mendoza the most, he said, was "the methodical manner in which the gunman . . . stood there and fired shot after shot after shot." Ashbrook was not spraying bullets indiscriminately, but was choosing his victims. "He was pacing a little bit back and forth," Mendoza said. "He is just holding his hand out with the gun in it, slowly, methodically picking things to shoot at."

Ashbrook's rampage, in which he also hurled a pipe bomb, ended when he sat in a rear pew and put a bullet in his head. By then the cameras had stopped taping, and three adults and four teenagers (three of them age 14) lay dead or dying.

One of the two videotapes was given to police by the person who recorded it. The other, taken from a camera found in the sanctuary after the shootings, may have been recorded by a victim. "We don't know yet," said Deputy Police Chief Don Gerland, who watched the tapes with Mendoza. At the point where each tape goes blank, Ashbrook is not pointing his gun toward either camera.

Ashbrook, unable to hold a job for more than a few weeks at a time, had shared a small house with his widowed father until the elderly man's death in July. Mendoza said Ashbrook was left with no financial support, and his mental condition, deteriorating for years, apparently began spiraling toward collapse.

After his father's death, Ashbrook displayed his growing paranoia in two rambling letters to the Fort Worth Star-Telegram and a visit to its offices, and in an hour-long phone conversation with a reporter for FW Weekly, an alternative newspaper. Ashbrook complained that he was being harassed at home, at workplaces and in social settings by police and government agents who, he said, suspected he was a serial killer. But he did not hint at violence, the newspapers said today.

"Unfortunately, you just can't look at these things and see danger," Managing Editor Kathy Vetter said in today's Star-Telegram. She said "99.99 percent" of such letter-writers are "just people who want to vent."

Pat Humphrey, the FW Weekly staff writer with whom Ashbrook spoke, said in an interview that he had sounded calm. "I really didn't think he was a danger," she said.

Mendoza said police had only one "contact" with Ashbrook--a marijuana arrest nearly 30 years ago. And he said investigators have found no record of his being treated or seeking help for psychiatric problems.

Police will not make the videotapes of the shooting public, Mendoza said. When investigators finish with them, they will be returned to their owners.

Mendoza and Gerland said many in the church initially thought Ashbrook's appearance was part of the evening's program--a surprise "skit" about evil. But they quickly realized the attack was not staged. Gerland said the videotapes of Ashbrook were taken by camera operators hiding on the floor between pews.

Ashbrook fired more than 30 shots. The tapes, Mendoza said, captured 90 seconds to two minutes of the rampage, in which Ashbrook squeezed off 20 to 24 rounds with the Ruger, which he had purchased legally for $325 at a flea market in February 1992. Ashbrook also had a .380-caliber semiautomatic handgun, purchased legally for about $150 from a different dealer at the same flea market on the same day. But investigators said they believe he fired only with the Ruger.

"I don't know if he hit on empty, but I imagine he did," Mendoza said. "He ejected a magazine, he loaded a magazine, and he continued firing."

Witnesses later quoted Ashbrook as shouting insults about religion, but his voice cannot be heard on the tapes over the din of the panicked crowd, Mendoza said.

"They were not a fast, short, quick sequence of shots," he said. "They were slow, methodical--he was picking, aiming and shooting."