CBS News Correspondent Bob Orr reports sources say FBI and Homeland Security agents are now checking out intelligence reports which suggest at least some of the five may be linked to terrorists.
The suspect pilots surfaced in July when US counterterrorism teams did new background checks on some 6,000 foreign pilots who fly for dozens of international airlines in and out of the US.
Officials say only that "something" in the backgrounds of five of the pilots raised enough of a red flag to create a concern that "they shouldn't be flying into the US."
A senior Homeland Security Official says the pilots "are not believed to be involved with any active plot to use aircraft to attack US interests."
However officials are still reviewing "second-hand" intelligence which has raised the possibility the pilots "might be affiliated with al Qaeda."
Investigators are concerned because captured al Qaeda leaders have made it clear that aviation remains both a favored weapon and a prime target.
But, some intelligence officials are downplaying the case, saying the pilots were banned not because of a clear threat but out of an abundance of caution.
The latest focus on Saudi nationals brings into sharp relief once again the debate - usually discussed publicly in the most diplomatic terms - over the fact that so many Saudis were part of the Sept. 11, 2001, terror attacks.
American counterterrorism officials believe the al Qaeda terrorist network primarily used Saudi Arabian men in the Sept. 11 attacks because they were easy to recruit and bring into the United States. But a Saudi official says Osama bin Laden sought out his countrymen to hurt the kingdom.
The Saudi official, who spoke on the condition of anonymity, said that bin Laden, who was born in Saudi Arabia, personally insisted that the attacks be carried out by a group mostly composed of Saudis.
The official said that suspected Sept. 11 mastermind Khalid Shaikh Mohammed has told his CIA interrogators that the original plan was to use a more diverse group, until bin Laden changed the plan to hurt Saudi Arabia and Saudi-U.S. relations.
CIA officials declined to comment on Mohammed's interrogation and would not confirm the Saudi official's statements.
But the notion that bin Laden, the al Qaeda leader, set out to hurt the Saudis with the Sept. 11 attacks doesn't hold much water in U.S. counterterrorism circles.
U.S. officials say al Qaeda drew heavily from Saudis because there were plenty who were willing to support the network's goals. Before the attacks, it was also easier for Saudis to receive visas to enter the United States than for people from some other countries.
Of the 19 hijackers, 15 were Saudis. But three of the four pilots - the Hamburg, Germany cell members thought to be the organizers of the attacks - were not. Mohamed Atta was Egyptian; Marwan al-Shehhi was from the United Arab Emirates and Ziad Jarrah was Lebanese.
The fourth pilot, Hani Hanjour, was Saudi, as were two other organizers, Khalid al-Mihdhar and Nawaf al-Hazmi. The rest, whom U.S. authorities describe as the muscle who helped take over the planes, included 12 Saudis and one man from the United Arab Emirates. Authorities believe these 13 were probably recruited by al-Mihdhar.
Two other alleged members of the Hamburg cell, Ramzi Binalshibh and Zakariya Essabar, sought flight training in the United States but were denied entry into the country.
Both are considered possible hijackers who ultimately played supporting roles in the plot. Binalshibh, captured a year ago in Pakistan, is Yemeni; Essabar, who remains at-large, is Moroccan.