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.
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.