WASHINGTON, Dec. 14— The Osama bin Laden videotape is raising new questions among intelligence officials about why American authorities failed to uncover the plot of the Sept. 11 attacks before they occurred.

On the tape, Mr. bin Laden said he knew six days in advance of the attacks the precise day and time of the hijackings and knew details, for example that there would multiple aircraft strikes.

At one point, Mr. bin Laden said he had turned on a radio that day to hear news of the attack. Speaking about his followers, he said, ''They were overjoyed when the first plane hit the World Trade Center, so I said to them, 'Be patient.' ''

The comment indicating precise advance knowledge of the attacks suggested that Al Qaeda's communications network, although heavily monitored by American intelligence, succeeded in circumventing the electronic eavesdropping arrayed against Mr. bin Laden and his followers. It also suggested that if Mr. bin Laden could receive information from the hijackers, he was also able to communicate his instructions to them without alerting the authorities.

A senior government official said that the passage on the tape that indicates Mr. bin Laden's prior knowledge of the timing of the attack had led intelligence officials to re-examine information collected in the week before the attacks to determine whether they missed a coded signal sent to Afghanistan from the United States. .

While the videotape released Thursday dispelled any doubt among investigators that Mr. bin Laden was the chief architect of the attacks, it has left unanswered questions that continue to perplex investigators. For example, there is no mention on the tape of the anthrax mailings, which some investigators have suspected might be the work of Al Qaeda.

Moreover, there is nothing on the tape to support the theory that other hijack teams were trained for other attacks on Sept. 11 or later, as investigators suspected. And there is no mention of the missing 20th hijacker or whether Zacarias Moussaoui, who has been indicted as a conspirator in the plot, was meant to be a hijacker.

Speaking at a unidentified location in Afghanistan, apparently last month, Mr. bin Laden revealed detailed knowledge of many operational details of the hijackings. He said that he knew the hijacking teams and how the teams were intentionally kept apart to avoid detection. He referred to Mohamed Atta, the Eygptian leader of the plot, as ''Mohamed.''

In his conversation with a Saudi sheik, later identified as Shiek al-Ghamdi of a Saudi province where some of the hijackers were recruited, Mr. bin Laden said that he was aware of the suicidal nature of the plot. He said that the hijackers also knew that they were being sent on a ''Martyrdom operation'' but did not know the precise plan until just before they boarded the doomed flights in Boston, Newark and Dulles Airport, outside of Washington.

One investigator said that the information might explain the report of a witness in Boston who told federal agents that he observed a group of men in a parking garage -- who the authorities later concluded were members of a hijacking team -- arguing violently just before the flights took off.