The independent commission probing the Sept. 11, 2001, attacks has found evidence suggesting the attacks were intended to be carried out in May or June of that year, but were postponed by al Qaeda leaders because lead hijacker Mohamed Atta was not ready, according to sources privy to the panel's findings.

New evidence gathered by the commission, including information obtained from U.S.-held detainees, indicates that Khalid Sheik Mohammed, mastermind of the attacks, persuaded al Qaeda leader Osama bin Laden to postpone the attacks by several months because of the organizational problems, according to the sources, who declined to be identified because of the sensitive nature of the commission's investigation.

That scenario would mark a dramatic revision of the commonly understood narrative of the Sept. 11 attacks and contrasts sharply with prevailing theories of FBI agents investigating the plot. Until now, federal investigators have said the evidence indicates that the attacks were likely planned for a narrow time frame around Sept. 11. If there had been an alternate date, investigators have said, it was probably later in the year.

The possible date postponement is included in a draft report that has been circulated among government and commission officials in recent days, and is expected to be among the topics discussed during a Wednesday hearing focusing on the origins and execution of the Sept. 11 plot. The panel, which is preparing to release its final report in late July, is expected to issue a separate report on Thursday exploring whether U.S. fighter jets may have been able to intercept American Airlines Flight 77 before it struck the Pentagon if they had been dispatched more quickly, according to commission sources.

Commission officials and members declined to discuss publicly the findings or evidence pointing to an earlier date for the hijackings.

Chairman Thomas H. Kean said the timing issue will be addressed, but he declined to comment on any conclusions. He said this week's hearings "will be two of the most interesting hearings that we've had, from the point of view of what we reveal about the plot and plotters and what we reveal about the response. . . . There will be new information."

One official who has seen the findings to be released Wednesday said they are based on "intelligence coming in that they wanted an earlier date. It's something really new."

Another official said the commission's conclusion appeared to be based in part on information gleaned from interrogations of Mohammed, who has been in U.S. custody since March 2003.

The new evidence indicates that the original timing of the attacks was postponed for readiness reasons and not in reaction to heightened security in the early summer of 2001, when the CIA, FBI and other agencies were on high alert for a possible al Qaeda strike, several sources said.

Bin Laden had been pushing for the hijackings to be carried out in May or June, but he was persuaded by Mohammed to agree to a delay because Atta and his conspirators were not prepared, one source said. The leading hijackers did not begin making reconnaissance flights for the hijackings until May, when they began flying transcontinental routes passing through Las Vegas, according to evidence compiled by FBI investigators.

The FBI has long believed that the hijackers were flexible about the date of the attack, but has not previously found credible evidence of an earlier date, according to law enforcement officials. Instead, some bureau investigators have focused on clues suggesting that the attack may have been moved up after the August 2001 arrest of alleged al Qaeda conspirator Zacarias Moussaoui in Minnesota, according to these sources, who requested anonymity because the evidence is classified.

"We've never had a theory that September 11 was supposed to occur earlier than September 11," one law enforcement official said. "There is a theory that it was supposed to be later but was moved up because of [Moussaoui's] arrest."

The subject of the date chosen for the largest terrorist attack in U.S. history has been a matter of widespread speculation and investigation since immediately after the strikes, which killed more than 3,000 in New York, Northern Virginia and Pennsylvania. Journalists and terrorism experts speculated widely on the possible significance of the date, noting, among other things, that it is the anniversary of the day Britain took over Palestine in 1922 under a League of Nations mandate and that "911" is the common U.S. emergency response number.

But FBI investigators and others who have examined the plot in depth have concluded that the evidence suggests the date was fluid, and was probably decided just weeks before the attacks were carried out.

For example, one senior law enforcement official said, before the 19 hijackers bought their tickets from Aug. 25 to Aug. 31, 2001, they spent days researching flights and tickets, according to records of e-mails and computer activity. The searches included many dates, not just Sept. 11, and included East Coast airports other than those used on Sept. 11.

The hijackers were looking for Boeing 757 and 767 jetliners, for which the pilots had trained and on which a half-dozen hijackers had flown as passengers in reconnaissance missions earlier in the summer. The only days they avoided in their research were weekends, the official said. Investigators have found no evidence that the hijackers bought tickets for any other planes on Sept. 11, nor have they concluded that other suspicious passengers were ticketed aboard other flights that day.

"You can see them looking for flights, but they're not looking for Sept. 11, and they're not only looking at Boston, Newark and Dulles," the official said. "It's not until they do all their research that they chose a date. They were not set on that Tuesday."

The issue of the date and how it was chosen is particularly important for many family members of the victims. On a different day, the passenger lists of the targeted jets would have been different, and many who were killed on Sept. 11 might not have been in their offices at the World Trade Center or Pentagon.

Kristen Breitweiser, a member of a group called the Family Steering Committee, said in an interview yesterday that evidence of an earlier date "will be a shock" to many relatives of those who were killed Sept. 11.

"This is an example of al Qaeda postponing something and carrying it through with great success," said Breitweiser, whose husband, Ronald, died at the World Trade Center. "This means they follow through, and I hope we learn from that."

Research editor Margot Williams contributed to this report.{grv}