Washington ? More than a year after the Sept. 11 terrorist attacks, the Senate voted overwhelmingly Tuesday to create an independent commission with broad powers to investigate how and why al-Qaida operatives were able to penetrate the nation’s defenses with such devastating results.

The 90-8 Senate vote came on a day when President Bush downgraded the terrorist threat alert one level, from “high” to “elevated” risk, with the government citing arrests of suspected terrorists from Lackawanna, N.Y., to Pakistan.

The increasing likelihood that a high-level investigative commission will be formed puts last year’s terrorist assault on the same status as the Japanese attack on Pearl Harbor and the assassination of President John Kennedy, both of which were scrutinized by independent, federally established panels.

The Senate voted less than a week after the White House dropped its longstanding opposition to an independent investigation, though family members of the Sept. 11 victims say they still fear the administration will find a way to undermine the commission’s fact-finding powers.

As for the terrorist threat level, it had been at code yellow, or “elevated,” since the system’s inception, but was raised to orange the second-highest level shortly before the anniversary of the Sept. 11 attacks. With the recent arrest of suspected terrorists around the world, officials said they believed the threat level could be downgraded.