ASHINGTON, Sept. 15 ・Two of the men believed to have hijacked the plane that crashed into the Pentagon on Tuesday were known to the authorities as associates of Osama bin Laden and had been sought in the United States since August by the Federal Bureau of Investigation, law enforcement and intelligence officials said today.

The two men, Khalid al-Mihdhar and Nawaf al-Hamzi, used their own names to purchase airline tickets and board American Airlines Flight 77, which left Dulles International Airport and slammed into the Pentagon.

Today, even as it appeared that the government had been unable to keep up with the two bin Laden followers, Attorney General John Ashcroft said the investigation was "developing a kind of clarity" as the authorities pieced together the lives and recent movements of the 19 men and their ground-based accomplices.

In fact, 25 people have been arrested on immigration violations as part of the investigation, a government official said today. None have been formally charged, either on immigration counts or with crimes relating to the hijackings of four planes that crashed into the World Trade Center and the Pentagon and in Pennsylvania, the official said.

Investigators have interviewed all of those in custody and some are cooperating with the authorities. Among the 25 are two men arrested at an Amtrak station in Fort Worth. They were interviewed by F.B.I. agents, taken into custody and flown to New York.

The men, Ayub Ali Khan, 51, and Mohammed Jaweed Azmath, 47, who were traveling to San Antonio, were removed from an Amtrak train in what authorities said was a routine drug search. The men were found to have box cutters and about $5,000 in cash. Hijackers aboard Tuesday's flights used box cutters and knives to commandeer the aircraft, officials said.

In the case of Mr. al-Mihdhar and Mr. al-Hamzi, intelligence officials said they alerted immigration authorities nearly two months ago that the men might try to enter the United States. But when immigration records were checked, officials learned that the men had already entered the United States. Both arrived at Los Angeles International Airport on Saudi passports, the officials said.

Mr. al-Mihdhar had been seen in January 2000 at a meeting of suspected terrorists in Malaysia that was under surveillance by American intelligence, officials said. After the bombing of the Navy destroyer Cole, intelligence analysts began to review possible suspects among the participants in the meeting.

In August, intelligence officials advised the Immigration and Naturalization Service to place Mr. al-Mihdhar on a watch list to bar entry into the United States, along with Mr. al- Hamzi, because the two men had traveled to the United States together in the past. The F.B.I. was alerted after officials learned they were already in the country, but the agency could not find them.

Last year, Mr. al-Mihdhar and Mr. al-Hamzi rented rooms from a retired professor in suburban San Diego, The Los Angeles Times reported today. The professor, Abdussattar Shaikh, told the paper that Mr. al- Hamzi had lived with him from September to December and that Mr. al- Mihdhar had shared the room in September.

Another of the men believed to be on the flight that crashed into the Pentagon, Hani Hanjoor, attended the CRM Airline Training Center in Scottsdale, Ariz., in 1996 and in December 1997. Each time, he tried to qualify for a private pilot's certificate and each time he failed.

"He'd be late; he wouldn't show up; he was unprepared; he didn't do his homework," said Paul V. Blair, controller of CRM. "He didn't seem like he was committed to being an airline pilot."

As the outlines of the plot began to emerge, law enforcement officials said they were disturbed by wide gaps in their knowledge of the scope of the operation, most significantly whether other hijacking teams might still be at large in the United States.

The officials said they had investigated a number of leads that hijackers were aboard other flights on Tuesday and failed to carry out their suicide missions or stayed on the ground preparing for a second wave of attacks.

So far, investigators have established that other men associated with the hijackers received flight training in Florida. Those men cannot be accounted for. Nevertheless, the officials said, the investigation has not produced concrete evidence that the terror plot included other targets.

Justice Department officials said today that they had obtained a second warrant in New York for a person wanted as a material witness in the investigation. Mindy Tucker, the Justice Department spokeswoman, said the warrant was issued by a federal court in New York under seal.

The first material witness was a man of Middle Eastern descent who was stopped on Thursday night at Kennedy International Airport with another person's identification.

The man, whose identity has not been released, was an associate of a brother of Osama bin Laden, officials said. The man was carrying identification documents belonging to his own brother. The man was also said to have used an address in the Boston area associated with people who may have been involved in the hijackings.

American and European law enforcement officials said today that they were examining whether any associates of Osama bin Laden may have tried to profit financially from the attacks by trading in put options or short-selling particular stocks, such as reinsurance companies or airlines.

Such kinds of trading are techniques for betting that a stock price would decline. American securities regulators said they had been contacted by their counterparts in Germany who were examining short- sales that occurred before the attack in three large European reinsurance companies whose stock plummeted inexplicably before the attacks.

Officials in Washington emphasized that they had no evidence of such kinds of insider trading but concluded that there should be a review of any significant and suspicious options trading or short-selling.

The authorities have distributed to other law enforcement agencies the names of 100 people, a list that includes suspected accomplices in the terrorist plot.

Out of the chaos of the attacks, several cities and regions in the country are emerging as focal points of the investigation, principally the cities where the flights originated or crashed and Florida, where a number of the hijackers received flight training.

Law enforcement officials in New York and northern New Jersey were tracing the movements made by several terrorists before Tuesday's attacks and examining possible support activities in the area, officials said.

On Wednesday, the F.B.I. searched the Marriott hotel at the Newark Airport, where some or all of the four hijackers of United Flight 93 are believed to have stayed on Monday night. The plane crashed in western Pennsylvania.

In Florida, where most of the hijackers are believed to have lived, particularly in recent months, F.B.I. agents spent three days searching a motel in Deerfield Beach where one of the suspected hijackers, Marwan al-Shehhi, apparently lived for two weeks before the attacks.

Richard Surma, the owner of the Panther Hotel, said that on the day before the attacks he noticed a black duffel bag in the trash. Inside the bag he found aeronautical maps of the East Coast, navigation equipment, books from a flight school, illustrated martial arts books, and a German/English dictionary.

Mr. Surma said the maps were "of the eastern half of the United States." The duffel bag also contained a computer printout that included a detailed list of airline flights and times, though Mr. Surma said he was not sure if they were departure or arrival times, or if they simply connoted flight patterns.

Mr. Surma said the F.B.I. also retrieved a box cutter from the room.

F.B.I. agents in New Jersey went to an apartment building in Jersey City to ask about the two men who had been seized on the Amtrak train in Texas.

The authorities believe that the men had lived at 6 Tonnele Avenue in Jersey City until recently. Residents said the agents broke into the apartment where the men appear to have lived.

Several residents said they knew little about the men. They said the agents arrested a Syrian truck driver who lived in the building and whose passport had expired. Other residents said they saw as many as four to five people leaving with the agents, though they did not know why.