The chatter at the Frederick Stamp Club rarely goes beyond talk about coveted and hard-to-find stamps. But a few months ago, two postal investigators joined the group's meeting to discuss a more pressing, deadly matter: anthrax.

The agents sat at a conference table in a Frederick County church and began to quiz club members, a mix of elementary school students and adults. They politely asked whether anyone had purchased a pre-stamped 34-cent envelope at a postal facility in the Frederick area, the type used in the series of mailings that delivered the lethal 2001 anthrax attacks.

"Unfortunately, none of us had purchased any envelopes in Frederick," said club member Jackson Cope, explaining that the agents wanted to know whether a certain blue-gray shaded envelope had been sold in the area.

It was an interesting night for the stamp club, but another in a long series of dead ends for law enforcement authorities. From stamp clubs to biowarfare research laboratories, the FBI and other agencies are continuing to seek the clues that will unlock the mystery surrounding the attacks that killed five people and sickened 17 others.

The spore-laden letters were mailed in pre-stamped envelopes to the offices of Sens. Thomas A. Daschle (D-S.D.) and Patrick J. Leahy (D-Vt.) and media outlets in Florida and New York. The people who died were two workers at the Brentwood Road postal facility in Northeast Washington, a Florida photojournalist, a New York hospital worker and an elderly Connecticut woman.

"We are going through and doing what we have to do to bring it to a resolution," said Michael A. Mason, head of the FBI's Washington field office. "We are working on this as actively as we did Day One."

About 30 FBI agents and 13 postal investigators are actively pursuing the case, dubbed "Amerithrax." In the past 33 months, agents have traveled to three continents and, according to the FBI, conducted more than 5,280 interviews, issued more than 4,480 grand jury subpoenas and contracted out thousands of hours of lab work, narrowing down the type of anthrax to a strain called Ames.

Yet the investigation keeps returning to the Frederick area, home to the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick. The lab, a possible source of the anthrax bacteria, once employed scientist Steven J. Hatfill, whose apartment in Frederick was searched twice by federal agents in the summer of 2002.

Described in August 2002 by Attorney General John D. Ashcroft as a "person of interest" in the probe, Hatfill is still viewed that way, according to law enforcement sources. They said they also have interest in a small group of other scientists, some of whom have no ties to Fort Detrick. The sources spoke on the condition of anonymity, citing the sensitive nature of the probe.

Hatfill, a medical doctor and bioterrorism expert, has not been charged with any wrongdoing. He has denied involvement in the attacks and has filed lawsuits against the Justice Department and the New York Times, alleging defamation.

The efforts in the Frederick area are being made as authorities try to finalize complex lab tests in hopes of tracing the anthrax to its point of origin. Scientists are hoping to match the gene sequence of the mailed anthrax or a contaminant found inside it to samples collected across the country and overseas, according to law enforcement authorities and other sources.

Scientists are continuing to refine the tests, which could take several months, law enforcement sources said. But the sources and scientists have cautioned that there are no guarantees that the tests will be of great benefit, because the nation's labs used to keep poor records of the whereabouts of anthrax stocks and who came and went at the facilities.

Fort Detrick, the largest employer in the city of Frederick, is one of several labs in the United States that have remained a focus of investigators. Early in the probe, investigators interviewed Fort Detrick's scientists; some were given polygraph examinations.

Last summer, the FBI spent about $250,000 and three weeks draining a pond in Frederick, acting on a theory that someone might have discarded contaminated materials there.

Although the pond yielded nothing that would aid the investigation, authorities have continued to look to Fort Detrick for leads.

Earlier this year, postal investigators interviewed employees at the Fort Detrick lab, according to their attorney, Rosemary McDermott. She said the questions included queries about access to "hot suites," areas at the facility where work is done on anthrax and other dangerous biological agents.

The investigators also wanted to know whether the employees ever saw anyone doing unauthorized research on anthrax, she said.

McDermott said the employees told investigators that they did not have access to the "hot suites" and that they never saw unauthorized research on biological agents.

In May, investigators re-interviewed another of McDermott's clients, Ayaad Assaad, a former Fort Detrick scientist. Assaad, an Egyptian American, was laid off in 1997 as part of a staff reduction and is suing the Army for age and racial discrimination.

Assaad, currently an acting team leader and toxicologist at the U.S. Environmental Protection Agency, was first interviewed in October 2001 after authorities received an anonymous letter identifying him as a "potential bioterrorist."

The letter, postmarked in late September 2001, was mailed after the first batch of anthrax letters was sent but before it became public that anyone had gotten sick. Assaad suggested in 2001 that someone responsible for the anthrax killings mailed the anonymous letter about him to divert attention.

Authorities have said they have no evidence suggesting Assaad was involved in the attacks.

Assaad and his attorney said they most recently met with agents at McDermott's office in Thurmont.

McDermott said the agents again asked questions about the anonymous letter and implied that it might be linked to the anthrax attacks. Agents also asked Assaad whether pre-stamped letters were available at Fort Detrick; McDermott said they were not.

In recent telephone interviews, Assaad recalled that agents asked him in May about the process of refining anthrax. He said he told the agents that it was not necessary to know that process, because the anthrax in labs was already in that form, and that there was a "long history" at Fort Detrick of people pilfering "anthrax and everything."

He said he then told agents that he suspected that current or former Fort Detrick employees were involved in the anthrax attacks and tried to make him a "scapegoat."

Assaad said he provided the agents with records from a conference in Crystal City that showed he was there when the first batch of anthrax letters was sent from New Jersey.

At one point, Assaad said, his attorney asked agents whether he was a suspect. "They said, 'Absolutely not.' They repeated that twice," he said.

Frederick Mayor Jennifer P. Dougherty said she expects that the FBI will remain a presence in her community in the months ahead, adding that the FBI "made it clear from the start that agents would be back regularly until the investigation is completed."

"I hope there is a resolution, but I hope the person comes from Poughkeepsie, not Fort Detrick," she added.

"We hope it's not someone who's a neighbor, either past or current. You don't want to be the one on the evening news saying, 'He seemed like such a nice guy.' "

The FBI drained a pond in Frederick last year but found nothing that shed light on the 2001 anthrax attacks, which killed five people.