Police investigating a fatal knife attack on an Upper East Side therapist questioned and released a Pennsylvania folk-singer who police sources said is considered the main suspect in the killing.

William Kunsman, 42, was questioned yesterday in a Pennsylvania police station about the death of Kathryn Faughey, a therapist who was hacked to death with a meat cleaver and two other long knives in her office this week. A 70-year-old therapist who shares her office, Kent Shinbach, was also attacked before the suspect fled and left behind two suitcases filled with knives, women’s clothing, duct tape, rope, and adult diapers.

A high-ranking law enforcement source said police had been led to Mr. Kunsman after discovering a series of recent e-mail messages he had exchanged with Faughey. Police sources said one of the witnesses to the incident had identified Mr. Kunsman, but the sources said police did not have enough evidence to arrest him yesterday. “There’s not enough to lower the boom on him,” a police source said.

The source said that after eight hours of questioning, Mr. Kunsman requested the representation of a lawyer after his wife alerted him that a news reporter was questioning her about the incident. He was released shortly after that.

In an interview yesterday with television reporters outside of a Pennsylvania police station near his home, Mr. Kunsman, who lives in Coplay, a town north of Allentown, Pa., denied that he was responsible for Faughey’s death. He wore a black sweatshirt and his thick, curly hair was disheveled. In surveillance video released by police on Wednesday, the man believed to be the attacker appeared to have a bald spot.

“They said they’d contact me if they needed me for anything, but I think my involvement is probably over,” he said of his interview with police.

Mr. Kunsman, a guitarist who has released an album of upbeat folk music, “Acousticology 101,” said he and Faughey had met in a guitar camp several years ago. Law enforcement sources said Faughey’s husband, Walter Adam, also attended the camp.

Mr. Kunsman said they had kept in touch since then, and he described Faughey as a close friend. “We just seemed to connect on a very deep spiritual level,” he said. “Kathryn was just a wonderful lady, and a fine person.”

Mr. Kunsman told the reporters that he was bipolar, and had consulted with Faughey about his illness. According t o Faughey’s Web site, she had specialized in relationship problems, especially those arising from the Internet.

Police sources said the e-mails were not sexual in nature, but did not characterize their contents further except to say they were not incriminating.

Mr. Kunsman said he learned of her death during his interview with police, and was shocked at the news. Mr. Kunsman’s wife, Lynn, told reporters yesterday that Mr. Kunsman had been home on Tuesday night.

“Finding out that somebody you’re very close to you has been killed is tough,” he said. “I think they were just doing their jobs. I’m very saddened about the incident.”

Faughey was killed shortly after 8 p.m. on Tuesday in the first-floor office of her psychology practice at 435 E. 79th St. Surveillance video in the building showed a man entering the lobby, walking briskly with a black bag and a large suitcase.

He asked a doorman to see Dr. Shinbach, the only other doctor in the medical suite at the time, and proceeded into the reception area of the doctors’ office. Inside the waiting room, he chatted with a patient who was waiting for Dr. Shinbach and at some point went into Faughey’s office.

Several moments later, Dr. Shinbach, who is a geriatric psychiatrist, and his patient heard screams coming from Faughey’s office. Dr. Shinbach ran to help, and was attacked as he entered the room. The ensuing struggled lasted for 10 minutes, ending only after the man pinned the doctor to a wall with a chair and stole his wallet.

Surveillance video captured the man leaving through a basement hallway, an hour after he had entered the building.

Faughey pronounced dead at the scene after she was stabbed 15 times, and Mr. Shinbach was recovering in hospital yesterday from numerous wounds.

The medical examiner determined that the cause of Faughey’s death had been a stab wound to the chest and blunt impact injuries to the head, a spokeswoman, Ellen Borakove, said.

High-ranking police sources said yesterday that they believed the suspect had come to the office planning to target Faughey, not Dr. Shinbach. Another police source said investigators were still hopeful that DNA, blood, and fingerprint evidence taken from the scene might yield evidence that could link Mr. Kunsman or another suspect to the incident.

A retired lieutenant from the police department’s Cold Case Squad, who is now a professor at John Jay College of Criminal Justice, Joseph Pollini, said the single witness who had identified Mr. Kunsman as the alleged killer could be enough to arrest him. As of yesterday, prosecutors had not issued an arrest warrant.

“That would be sufficient,” Mr. Pollini said of the photo identification, adding: “They’re going to tear apart that apartment with a fine tooth comb.”

The police were also going over the two suitcases left behind by the suspect when he escaped through a basement door. Detectives discovered yesterday that the diapers contained in the larger bag are not a brand that is readily available throughout the country. They are available in five stores in the vicinity of New York City, including Coplay, the town where Mr. Kunsman lives, law enforcement sources said.

Meanwhile, investigators filed in and out of Faughey’s office yesterday carrying about 20 separate pieces of evidence, including two large sofas, a small bag marked with a biohazard sticker, and three cardboard boxes filled with what police at the scene said were knives.