or Hani Hanjour, identified as the pilot who flew the jet that rammed into the Pentagon, blending into the American landscape began in Saudi Arabia with a $110 application for a four-week English course in California. He had only to prove that he had $2,285 to pay for the lessons, along with room and board. He never turned up for class.

Two other men the authorities said plowed jetliners into the World Trade Center, Mohamed Atta and Marwan al-Shehhi, entered the United States on tourist visas. Even without the required student visa, the men studied at a flight school in Florida.

Consular officers deluged with visa applications say they generally do not have much time to investigate the applicants.

Once foreign visitors enter the United States, immigration officers and law enforcement agencies usually have no idea if they are complying with the terms of their visas.

United States immigration officials said the hijackers exploited an immigration system that critics contend is riddled with loopholes.

Until Sept. 11, that system was geared to ease the way for commerce — whether in the form of tourism, business or study.

Experts on terrorism said security precautions often took a back seat to pressures from industry, the concerns of neighboring governments and even bureaucratic rivalries in the United States government.

According to the State Department manual for consular officers, participating in the planning or execution of terrorist acts would bar a foreigner from getting a visa, but "mere membership" in a recognized terrorist group would not automatically disqualify a person from entering the United States. Nor would "advocacy of terrorism."

The manual, apparently unchanged since Sept. 11, says that the United States will exclude immigrants who incite or direct terrorist activity, but that statements of a general nature that do not directly advance specific acts of terrorism are not automatically a basis for exclusion, however offensive the statements might be.

Some American investigators have said they believed that Mr. Atta, the apparent mastermind of the group, belonged to Egyptian Islamic Jihad, and that he met with Iraqi intelligence officials this year. Yet he apparently entered on valid visas and may even have re-entered the country after overstaying his visa on his last trip to the United States.

Zacarias Moussaoui, a Frenchman of Moroccan origin arrested last month after suspicious behavior prompted alarm at a Minnesota flight school, also entered the United States on a student visa, though the French police suspected him of terrorist ties. French intelligence officials shared this information with the F.B.I. after Mr. Moussaoui was arrested in August on charges of violating United States immigration law.

"In spite of elaborate immigration laws and the efforts of the Immigration and Naturalization Service, the United States is, de facto, a country of open borders," the National Commission on Terrorism said in a report last year.

The panel said the sheer volume of border crossings, 300 million by land from Mexico alone and millions more by air, made "exclusion of all foreign terrorists impossible."

In a prophetic warning, the commission noted that one of the terrorists involved in the first World Trade Center bombing in 1993 had overstayed his student visa. "Today, there is still no mechanism for ensuring the same thing won't happen again," the commission said.

The lapses appear to have greased the hijackers' way at almost every step of their conspiracy, from applying for visas to arrival in the United States, from the failure to track them once here, to their failure to leave the United States once their visas expired.

In the case of Mr. Hanjour, the school where he applied to study English, a branch of ELS Language Centers in California, had no way of knowing he had obtained his student visa, entered the country and gone AWOL.

Michael Palm, a spokesman for ELS, a division of Berlitz International, said the Immigration and Naturalization Service never sent the form documenting Mr. Hanjour's entry in the United States. Had it done so, the school would have filled out the form and sent it back, telling the service that Mr. Hanjour never reached the school. Instead, as far as ELS could tell, Mr. Hanjour belonged to its standard 10 percent of no- shows.

School operators say the immigration service faces a backlog of up to a year in notifying them of arriving students. And even if the school had gotten notice and relayed word of Mr. Hanjour's absence, the I.N.S. would probably not have considered finding him a high priority, immigration officials say.

Given the limits of its staff, the immigration service focuses on investigating crimes by foreigners, domestic smuggling and illegal workers, said Eyleen Schmidt, a spokeswoman for the I.N.S.

Two other hijackers, Mr. al-Shehhi and Mr. Atta, took five months of flight lessons at Huffman Aviation in Venice, Fla., paying the school more than $38,000, according to Rudi Dekkers, the owner.

Mr. Dekkers recalled that neither of the men carried student visas. Nevertheless, Mr. Dekkers said he believed there was no problem giving the men lessons that ended with their obtaining pilot's licenses.

Unlike foreign visitors working illegally, the two were "not making money, but spending it," Mr. Dekkers said. Immigration lawyers said Mr. Dekkers was correct: while visitors on tourist or business visas should first obtain student visas to attend flight school, it is not up to the school to ensure they do.

"I don't need anything from you, just a check to start flying," said Mr. Dekkers, who likened flying lessons to shopping for groceries. "We're just a business," he said.

A State Department official, speaking on condition of anonymity, said consular officials typically check the names of foreigners seeking visas each year against a database of four million names drawn from immigration, law enforcement and intelligence records. But the background checks are remarkable for what they do not include.

Officials at the Saudi Arabian embassy in Washington said that up to five of the hijackers carried stolen passports and false identities.

But unless an entire batch of passports vanishes from a government warehouse, countries do not typically report to each other on individual passports that may have been stolen or lost. As a result, American consulates around the world have no way of knowing whether the person presenting a passport is its authentic owner.

"There is no proactive check on whether the passport is stolen," the State Department official said. "It's extraordinarily rare that a foreign government would bring to our attention the theft of a specific passport."

Nor do State Department consulates have access to F.B.I. crime records open to any law enforcement and some regulatory agencies in this country, including the Animal and Plant Health Inspection Service. Despite appeals and task force recommendations in earlier years, the Federal Bureau of Investigation has so far refused to share those records with the State Department.

"It's been a turf issue," said one government official, predicting that the F.B.I. would be forced to give agencies not involved in law enforcement access to its criminal records. "I think that's something that they're not going to end up having a choice about," the official said.

After the World Trade Center bombing in 1993, Congress moved to tighten tracking of foreign visitors, including students. It ordered the immigration service to systematically match entries into the country with corresponding exits, for the first time.

It also ordered creation of an electronic databank on foreign students accessible to law enforcement officials. But both moves met with stiff resistance from business and educational institutions. And both were delayed.