A year ago this week, for no apparent reason, a high school senior named Tracey Lynn Kirkpatrick was stabbed to death by an unknown killer in the stockroom of a clothing store in Frederick, Md. Kirkpatrick had worked by herself that night. She was 17, a part-time clerk and an honors student headed for business college in the fall. The police were stumped.

Now, after months of intensive but mostly fruitless legwork by detectives, an unlikely new lead has suddenly recharged an investigation that appeared stalled. "To be honest with you, no, I couldn't have imagined it," Frederick Police Chief Richard Ashton said yesterday.

Someone used a pay phone in a Safeway supermarket near Frederick to dial a nationwide "confession hot line" in Las Vegas, a service that charges callers by the minute to tape-record their deep secrets for strangers who call and pay to listen.

"Hello," began the caller that night early last June. "My name is Don, and I'm calling from Frederick, Maryland. I know this is going to sound surprising, but three months ago I stabbed a girl to death."

He went on to give a few details. He said he had thought about surrendering to police, but decided against it. "Thanks for listening," he concluded. "I'm sorry for what I did, but nothing can change it. Bye."

An attorney for the hot line turned over a copy of the tape to Las Vegas police, Ashton said, and it was forwarded to police in Frederick, 45 miles northwest of Washington. This week, Ashton gave copies to four radio stations and asked them to broadcast the confession simultaneously at 9:15 a.m. Thursday, the first anniversary of Tracey Kirkpatrick's death. The stations complied.

Two hours later, someone called police and "positively identified" the voice, Ashton said. He said the name the caller gave was that of a man already considered a suspect, though not a strong one. Investigators approached him for the first time Thursday night. He declined to answer questions.

Police went back to his home with a search warrant shortly after 1 a.m. yesterday. Ashton said they took "items from his body," including a hair sample, and "items from his residence." He has not been charged in the case.

The "items from his body" were given to the Maryland State Police crime laboratory to be compared with evidence from the crime scene.

Results may not come for several weeks. The chief waits anxiously.

"Realistically," he said, "even if he did make that call, it doesn't mean he killed Tracey Kirkpatrick. It's important to keep that in perspective." Yet: "We hope everything we seized tightens a knot around his neck."

In a community of 40,000 people, the slaying of Tracey Kirkpatrick on March 15, 1989, "was such a tragedy, it struck everyone like it was a member of their own family," said Mayor Paul Gordon.

The third of four children, she lived with her parents in nearby Point of Rocks and was a few months shy of graduating from Brunswick High School.

As part of a work-study program, Kirkpatrick left school about 1 p.m. on the final day of her life and drove to the Westridge Square Shopping Center, where she worked as a clerk. At 4 p.m., she stepped next door to her other part-time job, at a women's sportswear store.

The store was supposed to close at 9. Just before 11, a security guard found the door unlocked, the lights on, and Kirkpatrick dead in the back room from numerous stab wounds. There had been no robbery, no sexual assault, and apparently no struggle. "No weapon, no motive," Ashton said.

"Why? Why? We're never able to put that question out of our minds," the girl's mother, Diane Kirkpatrick, said yesterday. "It's a question we want answered. Until then, it's hard for us to go about our lives."

Six detectives were assigned to the investigation. Everyone who knew the victim was interviewed. Her parents waited. Her Brunswick classmates planted a dogwood in her memory. Weeks passed, and eventually the stubborn case fell to one officer, Cpl. Barry Horner.

After the tape came, Horner told the Frederick News-Post, he listened to it repeatedly and is convinced the confession is genuine. Ashton said police were chasing other leads and he thought at first that releasing the tape would inundate them with useless tips. But he changed his mind recently.

The taped confession generated only eight calls, he said. Seven led nowhere.

"Hello. My name is Don and I'm calling from Frederick, Maryland. I know this is going to sound surprising, but three months ago I stabbed a girl to death. And you might think that in making this tape, I'm setting myself up to be caught, but there are a lot of guys named Don in Frederick. The girl I killed was working in a ladies' sportswear store. I often came by and talked to her when she was working alone, and one night when she was in the storeroom and we were talking, our conversation turned into an argument. And so I took out a knife that I have with me at all times and I killed her. And a few days later I realized that I had created a lot of sadness, and I thought about turning myself in to the police. But whatever they do to me, that won't bring Tracey back. So I've decided that I better keep free, because we have the death penalty in Maryland. Thanks for listening. I'm sorry for what I did, but nothing can change it. Bye."