US officials were given the first name and telephone number of an 11 September hijacker more than two years before the attacks, the New York Times has said.
Marwan al-Shehhi was a key member of the Hamburg cell
Quoting German officials, the newspaper says the CIA was given the name and number of Marwan al-Shehhi by German intelligence, who wanted him tracked.
They reportedly did not hear from the Americans until after the 2001 attacks.
A US commission into the attacks will investigate whether there was a failure to pursue the lead aggressively.
Marwan al-Shehhi is believed to have taken the controls of American Airlines Flight 175, which flew into the south tower of New York's World Trade Center.
According to the New York Times, German officials gave the CIA his first name and a telephone number in the United Arab Emirates in March 1999.
They had obtained the details by monitoring the telephone of Mohamed Heidar Zammar, a suspected Islamic militant in Hamburg, the paper said.
A senior German intelligence source said that after they passed on the information, they did not hear from the Americans until after 11 September.
"There was no response" at the time, said the official.
'Benefit of hindsight'
The paper quoted an American official as saying: "The Germans did give us the name 'Marwan' and a phone number, but we were unable to come up with anything. It was an unlisted phone number in the UAE, which he was known to use."
The Associated Press news agency was told by another unnamed US official that the intelligence community regularly receives thousands of full names of suspected terrorists, making them hard always to track.
The suspect piloted the plane that flew into the south tower
"A first name - and a common one at that - is a scrap of information and doesn't take you anywhere without the benefit of hindsight," the official said.
Marwan al-Shehhi was a member of the al-Qaeda cell in Hamburg, Germany, which is believed to have been at the heart of the 11 September plot.
A native of the UAE, he moved to Germany in 1996 and became a roommate of Mohammad Atta, who took over American Airlines Flight 11, which crashed into the north tower.
Philip Zelikow, executive director of the independent US commission investigating the attacks on New York and Washington, said in the New York Times they were actively investigating the issue.
"The Hamburg cell is very important," he said, adding that the intelligence on Shehhi, "is an issue that's obviously of importance to us, and we're investigating it."
But he stressed that no conclusions had yet been reached.