Elissa Gootman [New York Times]

GARDEN CITY, N.Y., Jan. 9  It started unfolding inside a psychiatrist’s office in a small brick office building here: one man confided to another that he wanted to kill as many as six people, then drop their dismembered bodies in the Atlantic Ocean, the authorities said today.

But the man who did the confiding, they said, was the psychiatrist, and his confidant was a patient he hoped to enlist in his plan.

Instead, the authorities said, the patient alerted the police, who arrested the psychiatrist, Dr. Richard J. Karpf, on Wednesday, after he tried to buy a gun with a silencer and 50 rounds from a detective posing as an underworld gun dealer.

Dr. Karpf told the undercover detective “that he wanted to shoot the people point-blank in the head and the heart,” said Fred B. Klein, chief of the major offense bureau of the Nassau County district attorney’s office.

Dr. Karpf, 50, of Great Neck, was charged with three counts of weapons possession. Today he pleaded not guilty in Nassau County First District Court in Hempstead. The authorities said that they were still investigating what they believed was a conspiracy to commit murder and that as the investigation progressed, they expected Dr. Karpf to be charged with more crimes.

Today, they said they had tapes of conversations in which Dr. Karpf filled his patient in on a grisly plot. “He told the patient allegedly that he had plans of killing these people all at one time,” Mr. Klein said, “of then dismembering the bodies, of putting the bodies in plastic bags, heavy-duty plastic bags, and that he wanted to rent a boat and get some cinder blocks to weigh down the body parts and take all this out on a boat in the Atlantic and dispose of the evidence in the ocean.”

The police said the conversations began about two weeks ago at Dr. Karpf’s office and elsewhere. The patient contacted the police about a week ago, they said, saying he feared others’ lives were in jeopardy.

Today, the authorities said they were still trying to determine who Dr. Karpf’s intended targets were. But Mr. Klein said they suspected that at least one potential victim might have been a patient.

Peter De Filippis, a Manhattan lawyer, said he believed that his client, a patient of Dr. Karpf’s, was among the targets.

The patient, a woman in her 30’s, had been seeking counseling from Dr. Karpf for the past year or so, Mr. De Filippis said, and the doctor had begun a sexual relationship with her in the past few months, in a breach of professional ethics.

Mr. De Filippis said the police told him that in outlining his plot, Dr. Karpf had “made statements indicating that he wanted to take care of a problem.” The police, he said, “strongly intimated to me that the problem might have been my client.”

On Tuesday, Mr. De Filippis said, his client, whom he would not name, showed up unannounced at Dr. Karpf’s office, at 1517 Franklin Avenue in Garden City, with plans to break off the relationship. But Dr. Karpf turned “irate,” Mr. De Filippis said, so the woman ran across the street to county police headquarters, seeking safety.

By then, the police were wrapping up their weeklong investigation, which culminated about 4 p.m. Wednesday in the parking lot of a Home Depot in Westbury.

The police said that Dr. Karpf, who had $1,600 in cash with him, mentioned his plans for killing people to the undercover detective, who had taken along a .22-caliber pistol with a silencer and four clips of ammunition, as the doctor had requested.

“It could probably best be described as an assassin’s weapon,” said Detective Lt. Dennis Farrell, commander of the Nassau County Homicide Squad.

Today’s arraignment, Dr. Karpf’s lawyer, Glenn H. Morak, asked Judge Martin Massell to set bail. Dr. Karpf, he noted, is a professional with no criminal record and deep roots in Great Neck, where he owns an apartment and has relatives. But the judge ordered Dr. Karpf held without bail. The doctor is scheduled to appear in court again on Monday.

In an interview, Mr. Morak described Dr. Karpf as a “hard-working person with a good practice.”

Mr. Morak added, “He’s a nonviolent person, and we believe that these allegations will not be borne out.”

In the tape of an interview with a reporter for WINS-AM (1010), a woman who insisted on being identified only as a relative of Dr. Karpf’s suggested that he may have tried to buy a gun for protection. “Every doctor, including a psychiatrist who would have mental patients, has to be protected in some way,” the woman said. “You can’t just allow yourself to be killed in an office.”

According to the American Board of Medical Specialties, Dr. Karpf graduated in 1980 from the medical school at the Universidad Aut¿noma de Guadalajara, in Mexico. He completed a residency in psychiatry at Bergen Pines County Hospital in Paramus, N.J. He is certified by the American Board of Psychiatry and Neurology and has been licensed to practice medicine in New York since 1983. A spokeswoman from the state health department said he had never been disciplined for anything.

Neighbors at 1 Kensington Gate, the four-story brick apartment building in Great Neck where Dr. Karpf lives, apparently alone, were taken aback by the news of his arrest. Many did not know him, but those who did described him as quiet and reclusive.

“I don’t think he said three words to me in 10 years,” said one neighbor, who did not give her name. “It’s sad he didn’t get help.”