his article was reported by Christopher Drew, Jo Thomas and Don Van Natta Jr., and was written by Mr. Van Natta.

WASHINGTON, Oct. 20 — After 40 days of the most aggressive criminal investigation in American history, federal law enforcement officials have arrested 830 people but have failed to develop evidence that anyone now in custody was a conspirator in the Sept. 11 terror attacks.

Despite pursuing more than 365,000 tips from the public, senior investigators in the United States acknowledged that most of their promising leads for finding accomplices and some of their long-held suspicions about several suspects have unraveled.

Beyond that, none of the nearly 100 people still being sought by the Federal Bureau of Investigation is seen as a major suspect, law enforcement officials said. "There is no one in that group that I'd want someone to wake me up at 3 o'clock in the morning to tell me about," a government official said.

Investigators' latest hope for a break was dashed late this week in Chicago. On Oct. 11, the arrests of nine Egyptian men living in Evansville, Ind., had sent ripples of excitement through law enforcement circles. Two senior law enforcement officials said there was speculation that the men were an Al Qaeda cell plotting a terrorist attack.

Federal authorities had learned that one of the men had recently taken flying lessons and that they lived together in Indiana and sent money home to Egypt, a law enforcement official said. The men were sent to Chicago for questioning, but on Thursday night, with investigators' suspicions evaporating, seven of the nine were released. One was still in custody and another faced only immigration charges.

One of the men released, Tarek Albasti, 29, a part owner of the Crazy Tomato restaurant in Evansville, had been arrested while making a pot of spaghetti during the dinner rush that night. As it turned out, the flight lessons that apparently made him appear suspicious were a gift from his father-in-law, a lawyer and former United States diplomat who is also a pilot.

The widespread arrests began the day of the terrorist attacks, and the numbers mounted as agents tracked down people through logs of the hijackers' cellphones, through interviews with their neighbors and through tips phoned in or sent to the F.B.I.'s Web site. But none of those arrested have been accused of playing a supporting role in the hijackings. Most are being held on unrelated immigration violations, traffic violations or charges of falsifying documents, prompting complaints from civil rights advocates and immigration lawyers.

But the arrests have a purpose beyond the investigation of the Sept. 11 attacks: to prevent more attacks. On that score, officials say the detentions have been much more successful.

Senior government officials say they believe they have captured at least 10 members of Osama bin Laden's network, Al Qaeda, who may have been involved in cells planning other attacks. While there is no firm evidence of specific plots that have been thwarted, one senior law enforcement official said, "We know we've changed the atmosphere."

Officials also say that they believe arrests in other counties since Sept. 11 have disrupted planned attacks.

Investigators say they know the hijackers received financial and logistical support from a small group of Al Qaeda lieutenants outside the country, and the investigation has increasingly focused overseas. But, one senior government official said, "We have seen no evidence of a big network here that helped them out."

The search for potential conspirators in this country has centered on nearly 20 material witnesses who are believed to have important information and have been brought to Manhattan for detention. But in recent weeks at least nine of them have been released from jail, officials said, and those still in custody are not cooperating.

"We are getting into squeeze time," a senior official said. "We are getting them before grand juries and confronting them with financial records and phone records, and it will be harder for some people to dodge and weave."

In the last 10 days, two men have been charged with lying to the grand juries investigating the attacks. One was charged in Phoenix with giving false statements to the federal investigators about his association with Hani Hanjour, the hijacker believed to have piloted the jet that crashed into the Pentagon. A Jordanian man attending college in California was charged with making false statements describing his association with two other hijackers, Nawaf Alhazmi and Khalid Almihdhar.

The charges allow the authorities to keep the men in jail while they continue to investigate. But law enforcement officials say there is no evidence either man had advance knowledge of the Sept. 11 plot. Investigators have established connections between the hijackers and about two dozen people now in custody, although those connections are only casual.

Shortly after the hijackings, the F.B.I. thought it had stumbled onto a cell of young Arab men in San Diego who might have helped two of the hijackers. The agency became interested in at least five men, most of them college students, through a tantalizing lead: the first name and the phone number of one of them, Osama Awadallah, was found scrawled on a piece of paper in the 1988 Toyota Corolla that Mr. Alhazmi, had left in a parking garage at Dulles International Airport in Virginia on Sept. 11 before boarding American Airlines Flight 77, which hit the Pentagon.

Federal agents learned that some of the men had roomed at the same Muslim leader's house in San Diego as Mr. Alhazmi and his fellow hijacker, Mr. Almihdhar. Some, including Mr. Awadallah, also had worked at a gas station where Mr. Alhazmi was briefly employed last year. And Mr. Alhazmi, as a favor, had included one of the students, Yazeed Alsalmi, on his auto insurance for two months in late 2000 to help him get a lower rate.

The F.B.I. arrested the five men and rushed them to New York to be held as material witnesses, prompting complaints from Randall B. Hamud, the lawyer for three of them, who said they were being tarred with guilt by association.

After being held for more than two weeks, Mr. Alsalmi, 23, a Saudi who attends Grossmont College in San Diego, was freed on Oct. 11 after testifying before a grand jury. But on Friday, Mr. Awadallah was indicted on charges that he had lied to the grand jury when he testified that he knew only one, and not both, of the hijackers who had come into contact with the students.

Mr. Hamud said the federal government was "arresting Arabs all over the country to make the public think they are doing something."

The strongest cases the federal government has made are against peripheral figures. For example, Luis Martinez-Flores was charged with being paid $50 each by two of the hijackers, Mr. Hanjour and Mr. Almihdhar, to help them obtain photo identification cards in Arlington, Va.

But as in the case of so many others, investigators originally believed Mr. Martinez-Flores was a significant witness. He had told investigators that the two hijackers were especially interested "in tall buildings in Richmond, Va., and Atlanta, Ga.," according to a confidential F.B.I. report. In particular, Mr. Martinez-Flores said, the men were "very interested in the Federal Reserve Bank in Richmond, Va."

He also said he had gone for a ride in their van, which contained suit caselike containers that he said "were extremely heavy and produced an irritating odor."

"When Martinez attempted to move the container, Almihdhar and Hanjour became concerned and took the container from him and cautiously placed it in a secure area of the van," an F.B.I. agent reported. "Martinez was advised that these containers had to be treated carefully." But two weeks after the interviews, prosecutors charged Mr. Martinez-Flores with lying to investigators about his experiences with the two suspected hijackers.

A government official complained, "A lot have been like that — they kick up some interest, and we look closer, and there's not much there."

At times, investigators have said they suspected that Zacarias Moussaoui, a French citizen of Moroccan descent who was arrested on immigration charges in Minnesota on Aug. 17, was a possible 20th hijacker. He was in jail on the day of the attacks. Investigators have not established that he was supposed to be part of the Sept. 11 plot, but they say they suspect he may have links to Al Qaeda.

Besides Mr. Moussaoui, investigators have focused on three other people with potential Al Qaeda ties: Nabil al-Marabh, a former Boston cabdriver whom an informer linked to Mr. bin Laden, and Ayub Ali Khan and Mohammed Azmath, two men who were carrying box cutters and at least $5,000 in cash when they were arrested aboard a train in Texas on Sept. 11.

Mr. Khan and Mr. Azmath had been on board a flight from Newark that was grounded in St. Louis. Investigators say they are still interested in both men, but they are not certain that they had anything to do with the Sept. 11 plot. Last week, investigators decided to test the Jersey City apartment where the two men lived in search of potential conspirators in the spread of anthrax.

Another potential cell that had intrigued investigators was in Chicago and Detroit. When agents raided a Detroit house at 2653 Norman Street on Sept. 17, they wanted to question Nabil al-Marabh. His name was on a mailbox, but he was gone.

Karim Koubriti, 23, answered the door. Two other men, Ahmen Hannan, 33, and Farouk Ali-Hamoud, 21, were also there. According to an affidavit by Robert Pertuso, the Detroit F.B.I. agent heading the investigation, the men told him that they had lived in the apartment for only two weeks and that Mr. Marabh may have lived there before.

Inside the apartment, agents found Skychefs Detroit Metropolitan Airport identification badges for Mr. Hannan and Mr. Koubriti, who had both once worked there as dishwashers, and a day planner with notations in Arabic. Mr. Pertuso said some of these related to the American base in Turkey; someone referred to as "the American foreign minister"; and Alia Airport in Jordan.

Agents say Mr. Koubriti told them that these documents, which also included passport pictures and false identifications, belonged to another man, Youssef Hmimssa, who had lived in the apartment. Mr. Hmimssa was arrested in Iowa. He, Mr. Koubriti and Mr. Hannan were indicted on charges of fraud and misuse of documents.

Law enforcement officials said they were still actively investigating the group in Detroit and Chicago.

The arrests of the nine men in Indiana angered their their families.

On Oct. 11, agents came to the Crazy Tomato restaurant and arrested Mr. Albasti, his uncle and other men who worked there.

The men were first taken to Kentucky and then to Chicago, where they were jailed in the Metropolitan Correction Center, Mary Frances Baugh, Mr. Albasti's mother-in-law, said on Friday.

"This really is a dark time for America," she said. "I don't think we need to increase the darkness by having the law stretched to these lengths by the attorney general." The treatment of detainees, she said, was "legal — but barely."

She said her husband had given her son-in-law flying lessons at Tri- State Aviation as a gift. "Once he got his license, he went on to get his instrument rating," she said. "My husband was so proud of him."

The F.B.I. first visited Mr. Albasti on Sept. 15, Mrs. Baugh said, and asked about his beliefs. "He told them there's a big difference between the Koran and peace and the lunacy of blowing up the World Trade Center," she said.

"I understand the terrible pressure the F.B.I. is under, because they have been mandated to never, ever let this happen again," Mrs. Baugh said. "At the same time, if America is not safe from its law enforcement agencies, if you can be scooped up fixing pasta and detained with no explanation and no communication, who among us is safe?"