Shortly after the first anthrax victim died in October, the Bush administration began an intense effort to explore any possible link between Iraq and the attacks and continued to do so even after scientists determined that the lethal germ was an American strain, scientists and government officials said.
But they said that largely secret work had found no evidence to back up the initial suspicions, which is one reason administration officials have said recently that the source of the anthrax was most likely domestic.
For months, intelligence agencies searched for Iraqi fingerprints and scientists investigated whether Baghdad had somehow obtained the so-called Ames strain of anthrax. Scientists also repeatedly analyzed the powder from the anthrax-laced envelopes for signs of chemical additives that would point to Iraq.
"We looked for any shred of evidence that would bear on this, or any foreign source," a senior intelligence official said of an Iraq connection. "It's just not there."
The focus on Iraq was based on its record of developing a germ arsenal and also on what some officials said was a desire on the part of the administration to find a reason to attack Iraq in the war on terrorism.
"I know there are a number of people who would love an excuse to get after Iraq," said a top federal scientist involved in the investigation.
From the start, agents searched for clues in domestic industry, academia and terror groups. But while investigators were racing to link the Ames strain to Iraq, they have only recently begun examining government institutions and contractors in this country that have worked with that strain for years.
In hunting for a culprit in the attacks that killed five people, agents have chased tens of thousands of tips in the past two months and conducted thousands of interviews, law enforcement officials said.
They have traced prescriptions for the antibiotic Cipro, on the chance the perpetrator took the drug to guard against the disease. They have also checked the language and block- style handwriting on letters sent with the anthrax against digital databases of threatening letters maintained by the Federal Bureau of Investigation, Secret Service and Capitol Police.
But officials said no likely suspects have emerged and they are settling in for what they fear could be a long haul.
The most promising evidence is still the anthrax itself, which federal scientists and contractors are studying for clues to its origin. The government tried to find links to Afghanistan and Iraq in the substance as well.
One discovery early in the inquiry seemed to undercut the foreign thesis. The anthrax used in the first attack, in Florida, and in subsequent attacks turned out to be the Ames strain, named after its place of origin in Iowa. While investigators found that this domestic variety of anthrax had been shipped to some laboratories overseas, none could be traced to Baghdad.
Nevertheless, government officials continued pushing the Iraq theory, scientists and officials involved in the inquiry said. They saw an intriguing clue in reports that Iraq had tried hard to obtain the Ames strain from British researchers in 1988 and 1989, raising suspicions that it had eventually succeeded.
Federal scientists hunted down records and biological samples from an investigation of Iraq's biological arms program, which was conducted by the United Nations in the 1990's. Those samples were analyzed in laboratories run by two biologists, Paul S. Keim of Northern Arizona University and Paul J. Jackson of the Los Alamos National Laboratory, in New Mexico.
But in the end few samples from Iraq's arsenal were found, and those that were turned out to have nothing in common with the Ames strain, officials said.
A different line of inquiry sought to re-examine seven anthrax strains that the world's largest germ bank, the American Type Culture Collection, in Manassas, Va., sold to Iraq in the 1980's, before the government banned such exports.
None of the strains were identified as Ames. But scientists inside and outside the government speculated that mislabeling might have inadvertently put the potent germ in Baghdad's hands. More laboratory tests were ordered.
Raymond H. Cypess, president of the germ bank, said recent investigations had disproved the mislabeling idea. "We never had it," he said of the Ames strain, "and we can say that on several levels of analysis."
The Iraq inquiry also looked for chemical clues. An early focus was bentonite, a clay additive that is one of the few substances identified publicly that can help reduce the static charge of anthrax spores so they float more freely and potentially infect more people.
Richard O. Spertzel, a retired microbiologist who led the United Nations' biological weapons inspections of Iraq, told investigators that Iraq had explored using bentonite in its germ weapons programs. But Maj. Gen. John Parker of the Army's biological research center at Fort Detrick, Md., said in late October that tests had turned up no signs of aluminum — a main building block of bentonite.
"If I can't find aluminum," General Parker told reporters, "I can't say it's bentonite."
Despite the scientific findings, the sophistication of the anthrax found in the letter mailed to Senator Tom Daschle, the majority leader, has kept Dr. Spertzel and others convinced that Iraq or another foreign power could be behind the attacks.
Richard H. Ebright, a microbiologist at Rutgers University who closely follows the anthrax inquiry, recently said that the Baghdad thesis "should not be dismissed as a desperate reach for a casus belli against Iraq" and is still worth investigating.
Publicly, White House officials have made no mention of the failure to find an Iraqi connection, but they have noted the inquiry's intensified focus on the United States. "The evidence is increasingly looking like it was a domestic source," the White House Press secretary, Ari Fleischer, said on Monday.
Tom Ridge, the director of homeland security, said in a statement that he initially assumed that the culprits were foreigners. "Like many people, when the case of anthrax emerged so close to Sept. 11, I couldn't believe it was a coincidence," Mr. Ridge said. "But now, based on the investigative work of many agencies, we're all more inclined to think that the perpetrator is domestic."
It remains unclear whether the focus on Iraq diverted investigators from the domestic inquiry. But some scientists say a decision made early on suggests that it might have.
In early October, the F.B.I. raised no objections when officials at Iowa State University, where the Ames strain was discovered, said they planned to destroy the university's large collection of anthrax spores for security reasons. Many biologists now say that step might have destroyed potential genetic clues to the culprit's identity.
Two months later, the investigation is largely focused in the United States. As the scientific inquiry into the anthrax itself continues, the F.B.I. is also employing more traditional forensic and investigative techniques to find out who sent the lethal letters.
Agents have compiled lengthy lists of who might have manufactured, tested, transported or stored anthrax. They have questioned manufacturers and marketers of biochemistry equipment and specialized machinery needed to make the material. They have inspected scientific literature, which could provide clues about who has knowledge to make anthrax.
But few clues have emerged. So far only three letters — those sent to NBC, The New York Post and Mr. Daschle — have been analyzed. A fourth letter, sent to Senator Patrick J.Leahy, Democrat of Vermont, is undergoing painstaking analysis by a number of laboratories, officials said.
All of the letters were photocopies and none appeared to contain any fingerprints. The plastic tape on the envelopes was a mass marketed variety. The paper on which the letters were written was an average size. The envelopes were prestamped and widely available. The marks left by the photocopier have been carefully studied, but have revealed no clues.
One senior official said nothing the investigators have found has led to anyone who might remotely be called a suspect. Several people who seemed to fit the F.B.I.'s profile of a science loner had been aggressively investigated, but no one has emerged as a serious subject.
"Still, the more you are out there, the more things bubble up," the official said. But asked whether recent news reports of a possible suspect in the case were true, the official replied, "I only wish that was true."
Some tips have seemed encouraging, but only for a time.
"We run out every lead and we give these people a real hard look and real hard shake before we take them off the screen," the official said. "There have been people who we have placed a little higher priority on than others." But then they fall off.
Some senior Bush administration officials have begun to worry privately that the case might take decades to solve, likening it to the Unabomber investigation that baffled investigators for nearly 20 years until David Kaczynski became suspicious of his brother Theodore and alerted the F.B.I.
Investigators have used various strategies to find suspects, but have often been frustrated. When they tried to track down people who had sought prescriptions for Cipro in the weeks before the anthrax mailings, the effort quickly bogged down. "Do you know how many people take Cipro in this country?" an exasperated official said, explaining that Cipro is used to treat a variety of ailments.
Investigators also said they were continuing to examine the possibility that the culprit might have purchased stock in the company that makes Cipro in an effort to profit from the attacks.
The newest front in the search for culprits is the examination of government research institutions and contractors. The reason to look there is plain: Some of them have the Ames strain and know how to turn it into the kind of deadly powder used in the attacks.
But that has added yet another complication to the already challenging inquiry. After all, investigators have relied on these same experts for scientific advice from the earliest days of the investigation, back when Iraq was a prime suspect.
"It puts us in a difficult position," one senior law enforcement official said. "We're working with these people and looking at them as potential suspects."