Moments before, Shannon Schieber's neighbor was sure he had heard the screams: "Help me! Somebody help me!" He had even knocked on the door across the hall: "Shannon? You all right? Shannon?" But no one answered.

Police banged on the door, too, and again there was no answer. He wondered if he'd really heard screams.

"I'll be embarrassed," the man told the officers, "if you break down the door and nothing happened."

"Call again if you hear something more," the officers told him as they left.

Investigators said today that they now believe the person who strangled Schieber, 23, may still have been inside her apartment early Thursday as the small group stood in the hall wondering what to do. Schieber's nude body was found later the same day. By then, the door to the balcony of Schieber's apartment was ajar. It hadn't been when the neighbor called police.

Homicide detectives said today they still had no solid suspects in the slaying of Schieber, who was killed in her second-floor studio apartment on a quaint alley called Manning Street.

Schieber had moved here last summer from Chevy Chase to begin doctoral studies at the University of Pennsylvania's Wharton School. She had been planning to go home for the summer, and Mother's Day, last weekend.

"She was very smart and worked very hard," her father, Sylvester Schieber, said in an interview Tuesday night. "The combination was a potent force. She could have done anything she wanted, and it probably would have made a big difference to somebody."

Sylvester Schieber is a vice president of the Bethesda benefits consulting firm of Watson Wyatt Worldwide and was on the federal government's special Advisory Council on Social Security.

His daughter, a 1992 graduate of Bethesda-Chevy Chase High School, was buried Tuesday in Montgomery County's Gate of Heaven Cemetery.

Inspector Jerrold Kane, of the Philadelphia police homicide division, defended the officers' actions today.

"Hindsight is 20-20," he said, "but it is not reality. You have to take each case and the totality of the circumstances at the time. You use common sense."

Although reports here have focused on a former boyfriend of Schieber's as the main suspect in the case, Kane played down police interest in him. On Feb. 25, Schieber had complained to security officials at the university about the man, a fellow student at Wharton.

Kane said she had told her family he was "stalking" her.

"He was certainly a guy we wanted to talk to, and we did," Kane said.

The man reportedly had professed his love for Schieber, who did not return it, and was later seen threatening her outside a classroom.

Schieber was frightened, and police said she had decided to resume her studies in the fall at Duke University, where she had been a distinguished undergraduate, rather than return to Wharton.

Kane said that the man had been very cooperative with police and that he is "not the target or the focus of the investigation."

Still, he said, the crime does not appear to be random, "not somebody just walking down the street." The intruder is believed to have entered the small alley and scaled a tree wrapped in barbed wire to reach the balcony of Schieber's apartment. Thirteen hours after the 2 a.m. call to police, Schieber's body was discovered by her younger brother, Sean, 22, and the same neighbor who had summoned police the first time, according to investigators.

Her brother had driven to Philadelphia to have lunch with his sister and pick up a friend, with whom he intended to travel to Amherst, Mass., for graduation at his alma mater, Hampshire College, Sylvester Schieber said.

He said his daughter, facing her last final exam Friday, told friends she planned to study until 4 a.m. Thursday, get some rest and then join others for more studying about 10 a.m.

She missed that appointment, and when she also missed the lunch date with her brother near the Penn campus in West Philadelphia, he drove to her apartment in the city's elegant Fitler Square neighborhood -- a community of stately brownstones and quaint row houses across the Schuylkill River from the university.

Police said that Sean Schieber got no answer at his sister's door and that he encountered the worried neighbor from the night before. They broke down the front door of her apartment after finding the balcony door open.

They found her unclothed body on her bed in an apartment that appeared to have been disheveled by a struggle, investigators said.

Today, the neighbor recounted what had happened and anguished over what he -- and the police -- might have done differently.

He said he had been up late watching TV, when about 1 a.m. he heard "a little commotion," a noise like books falling, followed by a faint exclamation from Schieber's apartment. Then, about an hour later, he heard distinct cries.

"It was striking enough at that point," he said. "I was convinced. I thought it came from her apartment, so I assumed it was her.

"I got up and knocked on the door," he said, and twice called out to Schieber, whom he said he knew casually. "Didn't hear a thing. That's when I started wondering, Did I really hear something?' It's just beyond my comprehension that I wouldn't hear anything more."

Schieber's family is mourning an accomplished daughter, who her father said had been a presidential scholar finalist, as well as a National Merit finalist. She had a 3.6 grade-point average at Duke, along with majors in mathematics, economics and philosophy, he said.

She also had taught economics to inner-city high school students in Philadelphia, her father said.

He said he last saw her more than two weeks ago when he was in Philadelphia for a conference. They were both busy -- she with finals -- and their visit was brief.

But that was okay, her father said he told her. "You'll be home in two weeks." CAPTION: Slaying victim Shannon Schieber, right, of Chevy Chase, at a wine tasting May 2 in Philadelphia. She had planned to transfer to Duke University, her alma mater, to continue her doctoral studies.