Case against bin Laden: how definitive?

They lived in groups of two or three, in nondescript rooms at places with names like Sandpiper Apartments, or Pin Del Motel.

Some were friendly. Some weren't. They were the type of people who might stop by an acquaintance's place for a cookie and a chat, but not stay long.

They ate pizza, bought sunglasses at local malls, used the weights at nearby gyms. A few had been in and out of the United States for years. The most remarkable thing about them may have been how unremarkable they were - until the last minutes of their lives.

Two weeks after devastating terrorist attacks struck the United States, the mammoth federal probe into who was behind them, and why, has made considerable progress.

Using a paper trail of plane tickets, bank records, and visa applications, investigators have developed a detailed picture of the 19 hijackers' loosely coordinated cells. Evidence made public suggests at least a circumstantial link with Osama bin Laden and his Al Qaeda terrorist network.

But officials haven't revealed any hard proof that the attacks were Mr. bin Laden's handiwork, either because they do not yet have it or because they are withholding it due to its sensitive nature. Their investigation has now entered the difficult phase of tracking terrorism's middle management: the intricate web of more senior operatives who may have provided cash and direction to attack cells.

"Just the amount and the extent of coordination and planning suggests there are higher-ups involved," says James Larry Taulbee, an Emory University political scientist and expert on terrorism.

Code named PENTTBOM, the FBI's bombing investigation is a massive law-enforcement response to the most deadly terrorist attacks ever. Directed from the agency's bunker-like main operations room beneath its downtown Washington headquarters, it involves some 4,000 agents and 3,000 analysts.

Its reach has already spanned the oceans. French authorities arrested seven Islamic militants suspected of anti-US activities in dawn raids on Sept. 21. Germany has issued arrest warrants for two similar suspects. Over the weekend, British authorities arrested three people in connection with the attacks. Spain and the Czech Republic are investigating recent and unexplained visits to their territory by a leader of the hijackers themselves - Mohamed Atta, an Egyptian thought to have been aboard American Airlines Flight 11 as it slammed into a World Trade Center tower.

Four potentially important material witnesses are already in US custody. (A material witness is a person thought to have information about the crime.) Two, identified as Ayub Ali Kahn and Mohammed Jawid Azmath, were arrested aboard an Amtrak train in Texas the day after the attacks. They were allegedly passengers on an American Airlines flight from Newark to San Antonio that landed in St. Louis after the FAA grounded aircraft on the day of the attacks. A third, Habib Zacarias Moussaoui, was already in jail in Minnesota on immigration charges, after attracting attention through suspicious activity at a local flight school.

The fourth, Nabil al Marabh, was nabbed in Chicago last week. Federal agents have said Mr. al Marabh is a former Boston cab driver known to have links to bin Laden operatives.

Overall, some 75 people have been detained in connection with PENTTBOM activity. The FBI has also compiled a "watch list" of 237 people sought for terrorism-related questioning in locations throughout the United States. Some of the key locations being looked at, however, are Florida, where 12 of the 19 hijackers may have lived, and the Detroit area, where three "watch list" men, including al Marabh, lived.

The number of people on that list and the number involved in the hijackings suggest the government is dealing with a different kind of terrorist in this attack than it has before.

Terrorist cells generally work independently in ways that are hard to trace. Foot soldiers carry out missions, but they are in the dark about larger plans. The scope of the Sept. 11 attack suggests cells were working in cooperation, involving more than just foot soldiers.

"There are indications that the those involved were pretty high up, higher up than we are used to seeing," says Peter Chalk, a security and terrorism expert at the Rand Corp. "They were pretty effective sleeper agents. They were here for some time. They took flight training. They had maps and diagrams of airports."

Indeed, many who knew the hijackers make a point of saying they were not particularly noteworthy at all. While some have said Atta was disagreeable and rude, the group largely blended in.

Rudi Dekkers, owner of Huffman Aviation, where two of the hijackers took lessons, says he checked Atta's "paperwork" and that of another suspected hijacker, Marwan al-Shehhi, and the only thing memorable was that they had spent time in Afghanistan, which meant nothing to him at the time. The hijackers' Huffman instructor, upon learning what his trainees did, was shocked. "He's so disappointed in himself," Mr. Dekkers says. "He thought after flying with them that he knew something about their character."

Some have been surprised to learned that the supposedly devout Muslims were seen at bars and even a strip club - practices some experts says are not unheard of with Muslims visiting America. One woman in New Jersey was startled to discover the man she had been dating was, in fact, Nabil al Marabh.

As the FBI digs deeper into the case behind the Sept. 11 attack, following the money trail and the string of relationships, the jolts may grow larger still. The hope is that somewhere in the surprises, the keys to the case - the elusive solid link to bin Laden and an understanding of the size and scope of his network in the US - become clear.

Lynn Waddell contributed to this report from Florida.