Since Feb. 8, when the 23-year-old woman died at her boyfriend's home here, a team of investigators systematically has explored thousands of leads in the hope of finding the person who turned an innocuous pain reliever into a merciless weapon.

They have found only dead ends.

They examined the smallest abnormalities. The FBI even compiled a dossier on a Tylenol assembly-line worker solely because he called in sick on the day that the news broke of Elsroth's death.

"I think it's safe to say we're no closer today than we were on Feb. 9," said Owen J. McClain, the Yonkers deputy chief of investigations. "We've eliminated a lot of people, that's all."

Now detectives from this community on New York City's north edge are looking to Camden County, where Louis Denber died on Sept. 1 after swallowing several spoonfuls of cyanide-laced Lipton Cup-a-Soup. Denber's death was the 11th homicide caused by a cyanide-tainted product in the United States. All of the killings are unsolved.

So far, the Yonkers investigators have not learned much from Denber's death. The cyanide that killed Denber was chemically different from the cyanide that killed Elsroth, a Westchester County prosecutor said last week. The method used to introduce the cyanide into the Cup-a-Soup was dissimilar as well.

In fact, the only similarity in the cases is the absence of details. Investigators in both cases believe the killers did not know their victims, made no threats and left no trails.

What Camden County investigators can learn from their colleagues in New York is the daunting task they face, a challenge not unlike searching in a crowd for a stranger, without the benefit of a description.

"When you have a serial rapist, when you have a serial murderer, you have all sorts of profiles you can draw on, all sorts of evidence that is left behind," a federal investigator said.

"But here, you have somebody anonymously poisoning products. We're up against someone different, a different crime that we have to learn about."

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Diane Elsroth's death on Feb. 8 was the first by consumer-product tampering since seven people died in Chicago in September 1982. Like the Chicago victims, Elsroth ingested cyanide-tainted Tylenol capsules.

Elsroth was a fetching woman with anthracite eyes who recently graduated from the C. W. Post Center of Long Island University, where she met Michael Notarnicola.

Both Elsroth and Notarnicola were from Westchester County - he lived with his family in an affluent section of Yonkers, and her family lived in Peekskill, in the rural, northern end of Westchester. They had been dating for three years when she died while staying at Notarnicola's house.

"Automatically, you assume it has to happen in the house," said McClain, the Yonkers deputy chief.

Police were immediately besieged with theories from numerous tipsters of dubious credibility, including "fortunetellers and psychics," McClain said.

"Everyone said the mother did it, or the father did it or the son did it," he said. "They said the mother was jealous of the son, the father had it out for the mother, the son wanted to kill the girl because she was pregnant."

Television crews camped out in front of the Notarnicola home while FBI agents questioned the family. A headline in the next day's newspaper said "Police Interview Yonkers Family," and the story stated that investigators had "focused their attention" on the Notarnicolas.

Police discovered that John and Harriet Notarnicola, Michael's parents, had argued several days before Elsroth died, but they discounted the dispute as minor. They found that the victim was not pregnant. None of the theories checked out.

Elsroth, who worked as a receptionist in New York City, often stayed in a spare bedroom at the Notarnicola home on weekends. At 1 a.m. on Feb. 8, a Saturday, she complained of a headache. Michael opened a new box of 24 Extra Strength Tylenol capsules that his mother had bought a few days before from an A&P in Bronxville, a cozy village next to Yonkers. He gave Elsroth two capsules. She went to her bedroom.

About 12 hours later, the Notarnicolas became concerned that Elsroth had not awakened and found her dead in her bed. Harriet Notarnicola was so distressed at the discovery that she took a Tylenol capsule - without ill effect. Investigators later found three other tainted capsules in the bottle.

Police investigated the backgrounds of anyone who knew the Notarnicolas, said McClain. They also investigated the Elsroth family.

"They have gone through everything," said Felicia Elsroth, Diane's mother. "Everybody who was a friend - everybody - was investigated."

Five days after Elsroth died, technicians at a U.S. Food and Drug Administration laboratory discovered the same type of cyanide in another bottle of Tylenol capsules, one that had been recalled after Elsroth's death from the F. W. Woolworth store in Bronxville, several blocks from the A&P where Harriet Notarnicola had bought the tainted Tylenol.

So investigators focused their full attention on the theory that the Tylenol contamination was a case of random product tampering. They said they never thought the Notarnicola family was involved.

"Unfortunately," McClain said, "as long as this remains unsolved, we may never absolve them."

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Likewise, investigators in Camden County discounted suicide or any family connections to Louis Denber almost immediately after the 27-year-old died from eating the contaminated Cup-a-Soup. Investigators made their decision on the basis of gut instinct - the absolute lack of a motive, along with what they viewed as the family's genuine reaction of distress.

Denber lived in Runnemede with his fiancee, Patricia Buhl, 25. He had life insurance, but Buhl was not the beneficiary, investigators said. The couple had a prenuptial agreement that would have divided their property after the marriage. She had a child by a previous marriage whom Denber had agreed to adopt.

"She had nothing to gain and everything to lose by his death," said one detective.

Denber's mother, Maureen, had purchased the soup for her son at the Clements Bridge Shop 'n Bag in Runnemede. But investigators found no reason why she or anyone else would want to harm her son.

In addition, Denber's mother reacted to her son's death in a manner that authorities say is not uncommon for the bereaved relative of a victim - she became a detective. She began calling RCA officials to inquire about Denber's co-workers at the same time that FBI agents and Camden County investigators were interviewing them.

Investigators say they will interview an expanding circle of Denber's acquaintances on the faint possibility that some enemy may turn up. But like the Yonkers investigation, most of their effort is now directed at identifying the cyanide that was found in another packet of Cup-a-Soup in Denber's home.

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Based on an analysis of the cyanide found in the unused capsules of Tylenol, investigators in Yonkers were able to conclude that it was different from that used in the Chicago killings. The cyanide contained trace amounts of silver that investigators said could have derived from the container that held it, or the cyanide could have been used in a manufacturing or photographic process.

Beyond that, they were unable to identify the manufacturer or the distributor of the cyanide.

Investigators interviewed retailers of cyanide and manufacturers who use it and compared hundreds of cyanide samples without a positive result, McClain said. "We tried that, but we gave up because it's so prevalent," he said. "There's no control on it. It's like buying a loaf of bread."

The FBI and the Securities and Exchange Commission compiled elaborate graphs of recent trading in the stock of Johnson & Johnson Co., the parent company of McNeil Consumer Products Inc., but found no significant trades that might indicate someone was trying to profit from the death by manipulating stock prices.

Because investigators did not immediately detect that the bottles of Tylenol containing cyanide had been disturbed - ever since the Chicago deaths, the manufacturer had been sealing the neck and the opening of the bottles - the investigation initially focused on the distribution process of Tylenol.

"It's not like a normal case where you'd ask a guy to establish his alibi," said Bruce P. Bendish, an assistant district attorney in Westchester County. "In this case, we don't know quite when it occurred."

Tracking McNeil's internal trail of records, investigators said they examined hundreds of people who could have come into contact with the Tylenol. The FBI ran background checks on employees at each point in the distribution chain, along with the truck drivers who transported the boxes of Tylenol.

The two bottles of tainted Tylenol were manufactured two months apart in two different McNeil plants - one in Fort Washington, Pa., and the other in Las Piedras, Puerto Rico. Investigators closely regarded workers at a McNeil warehouse in Montgomeryville, Pa., through which the bottles passed within two weeks of each other.

The FBI conducted a thorough exam of an employee at the Fort Washington plant who called in sick on the day that Elsroth's death was announced, on the hunch that he suffered from guilt. Likewise, they investigated a chemist at that plant who had been reprimanded two weeks before the capsules were made. They interviewed an assembly-line worker with a minor criminal record at the Puerto Rican plant.

At one point investigators discovered that a group of about 20 corporate McNeil employees had visited the two plants, two months apart, on the days when the Tylenol capsules were manufactured. All of the employees came up clean, Bendish said. None of them had been near the assembly line.

"I don't think there was anything we ever had when we thought we were well down the road to solving it," he said. "The investigators were so thorough, that as fast as something appeared, it was ruled out because of their investigations."

In their foraging, investigators pursued the remotest hunches.

For instance, authorities briefly considered James E. Lewis a suspect. Lewis was serving a prison sentence for sending an extortion note to Johnson & Johnson in the Chicago Tylenol deaths, but was not charged with the killings. Investigators said Lewis' wife lived in Boston, and they considered whether the contaminated Tylenol might have been meant for her. But they quickly dismissed the idea.

Then, a few days after Elsroth's death, police arrested a 21-year-old New Rochelle, N.Y., man on charges of credit-card fraud and stumbled upon an extortion letter in which he claimed he was "Tylenol Killer No. 2" and sought $2 million. The letter had not been mailed. Bendish said investigators dismissed the man as a suspect after concluding he was not intelligent enough to commit the tampering.

"We never close an investigation," McClain said. "We just inactivate it, which is just a semantic difference. We've pretty much exhausted all avenues."

McClain initially assigned 30 of the 50 Yonkers detectives under his command to the federal task force investigating the Tylenol tampering, but that number has dwindled. He said he would decide this week if the two remaining detectives investigating Elsroth's murder should be reassigned.