Investigators are intensifying efforts to trace the last encounters and daily routines of a New York hospital worker before she died of anthrax last week, hoping to unravel the mystery of who is behind the attacks that killed her and three other people and sickened a dozen more.

The focus remains on 61-year-old Kathy T. Nguyen, according to senior federal law enforcement officials, because investigators believe her habits or relationships may take them somewhere other than the routes of three anthrax-tainted letters mailed from Trenton, N.J., a trail that baffled investigators seem to feel has grown frustratingly cold.

"We're missing something," a senior government official said. "There's something wrong here."

Of the anthrax cases, Ms. Nguyen's stands alone in defying comprehension. She contracted a lethal dose of inhalation anthrax, but no traces of the bacteria have been found anyplace she is known to have been in her last few weeks or on any item of clothing she might have worn.

Ms. Nguyen died Oct. 31, three days after checking into a Manhattan hospital.

"Nothing in her house. Nothing at work. Nothing in her mail. Nothing anywhere," said a senior law enforcement official.

Unpersuaded for the moment that Ms. Nguyen developed the disease from mail cross-contaminated in some fashion by contact with the known anthrax letters, law enforcement officials said it remains possible she actually crossed paths with whoever unleashed the attacks. But where and when that may have happened, or with whom, remain open questions.

That theory is partly a hope built on last-ditch optimism, because the alternative — that some innocent letter addressed to her was cross- contaminated — would leave the investigation essentially where it has been almost from the start: nowhere.

Investigators admit that it has been difficult pinpointing everything Ms. Nguyen did in her last days. Did she buy gum here, get coffee there? One momentary encounter might have been the only one that matters.

Epidemiology is a field that learns from patterns, but Ms. Nguyen's case is stubbornly devoid of patterns — no traces of spores in her environment, no obvious correlation to the known germ-laced letters, no emergence of related cases that point in a direction.

Every day, detectives and medical investigators assemble before wall charts that represent the hospital worker's final weeks. Every day, they hope to fill in more of the chart's gaps. But most of the blanks remain just that: blank, save for the long hours and frequent double shifts she worked at the Manhattan Eye, Ear and Throat Hospital on the East Side of Manhattan, according to Dr. Marcelle Layton, the New York City assistant health commissioner.

Still, each day investigators delve anew into those aspects of a structured and quiet routine, looking for some light that has yet to shine. Again and again, a discovery about her life raises hopes only to later deflate them. In one case, investigators were excited to find a receipt for an airline ticket in her apartment. Then they noticed its date — 1991.

In trying to divine the details of an encounter that caused the exposure, agents of the task force have tried to interview anyone who might known anything about Ms. Nguyen. Father Carlos M. Rodriguez, the pastor at St. John Chrysostom Roman Catholic Church on 167th Street in the Bronx who presided over her funeral yesterday, said investigators visited him looking for details. All he was able to tell them, he said, was that she often attended his 10 a.m. Sunday Mass, and that she also worshiped at a Catholic church in Midtown. He wasn't sure which one. What's more, he told the authorities that she liked to shop for groceries in Chinatown, but he didn't know which shops.

Dave Cruz, the superintendent at the building at 1031 Freeman Street in the South Bronx, where Ms. Nguyen lived on the third floor, said he was interviewed by agents at least twice, once shortly after he dropped her off at Lenox Hill hospital when her illness worsened and again after she was diagnosed with anthrax. He wouldn't discuss what she said to him during the trip to Lenox Hill, and said he knew little of her routines. "That's the mystery right there," he said. "Other than being at work and home, I don't know where she went."

Anna Rodriguez, who lives above Ms. Nguyen's apartment, said that agents have been questioning everyone who showed up at the Ortiz Funeral Parlor in the Bronx on Saturday and Sunday, when Ms. Nguyen's body was laid out for viewing.

"It's a tough one," said Jerome M. Hauer, former director of the city's emergency management office. "It's almost the way you try to find out who murdered her. Who did she make contact with? Where'd she spend her last days?"

As Ms. Nguyen was buried yesterday, and as Manhattan Eye, Ear and Throat announced that it would reopen today, a few fresh tidbits about her surfaced. One lead that investigators are pursuing is that Ms. Nguyen, who worked in a stockroom in the hospital's basement, may have moonlighted at a restaurant. It was not clear, however, whether officials knew of a specific establishment, or whether Ms. Nguyen worked there or simply ate there.

Dr. Bradley Perkins, an anthrax expert at the Centers for Disease Control and Prevention, said in a conference call with reporters yesterday that, "We do not have any very good leads as to where or how the exposure occurred."

Dr. Bradley said that, amazingly enough, even as many more samples have been tested from the hospital, her apartment, her car, along her daily subway route to work on the No. 6 train, and from other places she was known to have visited, there has not been a single positive reading for anthrax. Investigators have been running swabs over every sweater, tabletop and doorknob that Ms. Nguyen might have touched, rubbed or passed.

Authorities concede that anything is possible, including that Ms. Nguyen did in fact touch a contaminated letter. But if that is true, it would be one more surprise in a succession of anthrax surprises over recent weeks.

From what is known, Ms. Nguyen's case doesn't fit the cross- contamination pattern. Four people have died after developing the disease in their lungs — the three others being a photo editor in Florida and two postal workers in Washington — and the same strain of anthrax was identified in all four cases. But in the three other deaths, anthrax was detected at their offices and presumed to have originated in a letter.

Ms. Nguyen's path is not believed to have crossed news media outlets or postal facilities.

She first became seriously ill on Oct. 25. Dr. Perkins said the likelihood was that she was exposed no more than four or five days before then, or around Oct. 20. This is well beyond when the known anthrax letters arrived in New York. Also, other cases of anthrax triggered by cross contamination of letters have been limited to a less-severe form: cutaneous, or skin, anthrax. Given these factors, Dr. Perkins said, it is unlikely that Ms. Nguyen was infected through cross-contamination.

The greatest hope of investigators is that she somehow intersected with one or more people responsible for the attacks.

Frustrated at their inability to achieve a breakthrough, officials have told the F.B.I.-N.Y.P.D. Joint Terrorist Task Force to continue their dissection of her routine, and investigators are believed to be intent on formulating as detailed a biographical portait of Ms. Nguyen as they can.

Their work has been hindered by the fact that Ms. Nguyen, a Vietnamese refugee who arrived here in 1977, died without being able to be interviewed about her life and routines. She lived alone and had no nearby family. Indeed, at her funeral yesterday, it was said that she might have lost almost all of her immediate family during the war in Vietnam. Neighbors in the Bronx depicted her as friendly and unremarkable, but it was unlikely that anyone knew with precision her itinerary in the weeks preceding her death.

Even a $1 million reward for information, which produced a spate of telephone calls and e-mail messages to federal law enforcement agencies over the weekend, has so far left the anthrax investigation starved for promising tips.