U.S. authorities missed some obvious signs that might have prevented some of the Sept. 11 (searchhijackers from entering the country, the federal commission investigating the attacks said Monday.

The bipartisan National Commission on Terrorist Attacks Upon the United States (search), facing a May 27 deadline to issue its final report, also agreed to formally ask Congress to extend the deadline by at least two months.

The move sets up a potential standoff with the Bush administration and House leaders, who have opposed a delay that could push the report's release into the height of the presidential election season.

The commission will announce its plans to seek an extension at Tuesday's hearing and will make the request of Congress sometime this week, according to a person familiar with the commission who spoke on condition of anonymity.

Family members of those who died in the attacks have said they support an extension of the deadline because the 10-member commission has been bogged down by disputes with the administration and New York authorities over access to documents and witnesses.

At the hearing Monday, the commission noted that while government officials have said the 19 hijackers entered the country legally, its investigation found at least two and as many as eight had fraudulent visas. The commission also found examples where U.S. officials had contact with the hijackers but failed to adequately investigate suspicious behavior.

For example, Saeed al Ghamdi (searchwas referred to immigration inspection officials in June 2001 after he provided no address on his customs form and only had a one-way plane ticket and about $500. Al Ghamdi was able to persuade the inspector that he was a tourist.

The panel also found that six of the hijackers violated immigration laws by overstaying their visas or failing to attend the English language school for which their visas were issued.

And Khalid Sheikh Mohammed (search), believed to be the mastermind of the Sept. 11 attacks, exploited the fact that customs officers did not routinely collect fingerprints for a visa even though federal authorities in New York indicted him in 1996 for his role in earlier terrorist plots. He never entered the country and was apprehended after the attacks.

Four of the hijackers' passports were recovered, including one found on the street minutes after the plane he was aboard crashed into the north tower of the World Trade Center and before the New York landmark collapsed.

At the start of a two-day hearing on border and aviation security, the commission staff issued a statement saying FBI Director Robert Mueller had testified that all of the hijackers came "lawfully from abroad," while CIA Director George Tenet described 17 of the 19 hijackers as "clean."

"We believe the information we have provided today gives the commission the opportunity to reevaluate those statements," the commission staff said.

The panel said part of the problem was a lack of coordination among immigration officials and a focus on keeping out illegal immigrants rather than potential terrorists.

The bipartisan panel was created by Congress to study the nation's preparedness before Sept. 11 and its response to the attacks, and to make recommendations for guarding against similar disasters.

It has held six hearings to gather information. Among those it heard from Monday was customs agent Jose E. Melendez-Perez. He said that suspected Sept. 11 ringleader Mohamed Atta raised enough red flags — including having the wrong student visa — that he should been blocked from entering the United States.

He explained that Atta's age and impeccable clothes appeared to contradict his story about being a student. "I would have recommended refusal," said Melendez-Perez.

Melendez-Perez is credited with stopping a man who U.S. officials believe may have planned to be the 20th Sept. 11 hijacker.

The man, identified by federal officials only as al-Qahtani, was stopped at Florida's Orlando International Airport in late August 2001. Melendez-Perez said he became suspicious when al-Qahtani provided only vague answers about what he was doing in the United States.

U.S. officials then put al-Qahtani on a plane back to Saudi Arabia. He wound up in Afghanistan, where he was captured by U.S. forces. He now is being held with other captives at the U.S. naval base at Guantanamo Bay, Cuba.

The commission detailed other government missteps prior to September 2001:

—Three of the hijackers, al Ghamdi, Khalid al Mihdhar and Hani Hanjour, submitted visa applications with false statements about never previously applying for a visa that could have been easily verified.

—One hijacker, Ziad Jarrah, entered the United States in June 2000 on a tourist visa, and then enrolled in flight school for six months. He never filed an application to change his status from tourist to student. Had immigration officials known, they could have denied him entry on three subsequent trips.

Mary A. Ryan, former assistant secretary for consular affairs at the Department of State, said the nation's visa processing system was hindered by insufficient data from intelligence officials about suspected terrorists as well as a lack of staff, which limited lengthy questioning of suspects.

"Any name check system is and will be only as good as the information that is in it," Ryan said, acknowledging under questioning by commissioners that information-sharing remains poor among federal agencies.

Tuesday's hearing will focus on vulnerabilities and security failures within the nation's aviation system.