ASHINGTON, Sept. 25 ・About 20 of the people arrested after the Sept. 11 attack on the World Trade Center and the Pentagon had obtained or tried to obtain special driver's licenses allowing them to transport explosives, poisons or other types of hazardous materials, federal officials said today.

The authorities are investigating whether these men may have been part of a terrorist plot. But they said they did not have evidence that they were part of the plot to hijack the airliners used to attack the World Trade Center and the Pentagon.

The arrests prompted the F.B.I. to issue warnings today to officials in New York and other cities about the threat of truck bombs or the use of trucks in a chemical attack, the officials said. In response, New York City officials stepped up spot inspections of trucks at the tunnels and bridges leading to Manhattan.

Attorney General John Ashcroft, testifying today before the Senate Judiciary Committee, said those arrested "may have links," which he did not specify, to the 19 men suspected of hijacking four jetliners on Sept. 11. Another senior official said the efforts to obtain special trucking licenses suggested "that these people may have looked at a broader range of ways to attack us."

Mr. Ashcroft did not detail the possible connections. Other officials said they had not yet determined whether the hazardous materials licensees were coordinating another attack with the suspected hijackers, operating in an alliance with them, or had no connection to the plot.

On Monday, a senior law enforcement official said the Federal Bureau of Investigation had not yet found any knowing accomplices to the suspected hijackers in the United States or any broad network of support in this country for that plot. The official said the F.B.I. was investigating whether the hijacking plot may originated in Germany, but was continuing to look for accomplices in the United States.

As the authorities compiled information about people they sought in the hijacking investigation, they began finding many of these license applications and began to see a pattern that remains unexplained. Because few of the men in custody are cooperating, it is possible that investigators may never fully understand whether, as some officials suspect, they unearthed preparations for some other kind of attack.

Today, officials would not identify any people in custody who got or applied for hazardous materials driver's licenses, nor would they detail how these men might be linked to Osama bin Laden nor why they had first come to the attention of authorities in the hijacking investigation.

Mr. Ashcroft compared the authorities' interest in these 20 men to their investigation of reports that Mohamed Atta, who is suspected of being a central figure in the hijacking plot, had asked suspicious questions about crop-dusting planes.

The 20 men were arrested since the attacks on suspicion of lying about their identities on applications for the licenses, law officials said.

The men are not cooperating with investigators, the officials said. Their efforts to obtain these licenses were detected after several of them were arrested as part of a broad sweep of people that the F.B.I. believed might have had some connection to the hijackers.

States issue the special driver's licenses, but the minimal standards are set by the federal government. The licenses permit people to drive trucks that carry poisonous, flammable, or other dangerous agents.

Officials said the 19 suspected hijackers did not have licenses to transport hazardous materials.

Heavy trucks have been used before in some of the most deadly terror bombings that have been attributed to Mr. bin Laden's followers.

A Nissan Atlas refrigerator truck packed with explosives was used in the August 1998 bombings of the United States embassies in East Africa that killed 224, including 12 Americans. Mr. bin Laden was indicted as a conspirator in that attack, and four of his associates were convicted in the plot.

A rental truck packed with explosives was used in the 1993 bombing of the World Trade Center.

In his comments to the Senate Judiciary Committee, Mr. Ashcroft said that the F.B.I. had informed the local police to be vigilant for suspicious activity regarding hazardous materials and crop-dusting aircraft. But he did not say whether the authorities had specific information about a continuing threat.

Today, the Transportation Department issued a warning to transportation companies of unconfirmed reports of the possible attacks involving chemical, biological or radioative materials. The warning said in part, "The F.B.I. and U.S. Department of Transportation have no information of any specific threats directed against additional targets; however, the U.S. transportation industry should remain at a heightened state of alert."

Last week, federal agents in Detroit who were searching for Nabil al-Marabh, a suspected associate of Mr. bin Laden, uncovered evidence that he obtained a license to transport hazardous materials.

The authorities had previously linked Mr. al-Marabh to a former Boston cabdriver now jailed in Jordan on charges of plotting attacks on tourist and holy sites there on the eve of the millennium celebrations. The attacks were foiled by Jordanian authorities, who said the plans were ordered by one of Mr. bin Laden's top lieutenants.

Since the attacks on Sept. 11, F.B.I. agents have questioned former friends and associates of Mr. al-Marabh, 34, including drivers at the Boston Cab Company, where he worked.

In Michigan, Mr. al-Marabh presented an immigration card with a photo and driver's licenses from Ontario and Massachusetts to obtain a chauffeur's license, said Elizabeth Boyd, a spokeswoman for the Michigan secretary of state. On Sept. 11, 2000, Mr. al-Marabh upgraded the license so he could drive 18-wheelers and transport hazardous materials.

States issue licenses to commercial drivers under federal rules. Each driver must pass a driving test and a written examination. Drivers may upgrade their licenses to permit them to transport hazardous materials. To upgrade their licenses, drivers are tested on their knowledge of hazardous materials, understanding of safety measures and transportation methods for various materials.

The Federal Motor Carrier Safety Administration at the Transportation Department sets standards for commercial drivers. They must submit information including their true name, a photo and valid address.

In Florida today, a bank president said F.B.I. agents told him that one of the suspected hijackers sought a government loan last year to buy a crop-duster plane or set up a crop- dusting business.

Robert Epling, president of Community Bank of Florida, in Homestead, said federal investigators told bank officials that Mr. Atta went to a Farm Service Agency office of the Department of Agriculture on the fourth floor of the bank's building to apply for a loan about a year ago. Mr. Epling said three employees later identified Mr. Atta. After telling him their office did not make such loans, Mr. Epling said, they referred Mr. Atta to the bank on the first floor.