Washington ? Anti-aircraft missiles guarded Washington’s skies, and Capitol police carried gas masks Wednesday as the nation mobilized to confront a potential terrorist attack.

Federal, state and local governments tightened security, anxious Americans stockpiled food and water, and police responded to scores of false alarms, including reports of suspicious vehicles that shut down commuter bridges in Washington and New York.

The nation remained under a Code Orange “high risk” of attack status for a sixth day, and no change was in sight. Counterterrorism officials said the level of threat information pointing to an imminent attack remained high, but steady.

“If given the choice, al-Qaida terrorists will choose attacks that achieve multiple objectives, striking prominent landmarks, inflicting mass casualties, causing economic disruption and rallying support through shows of strength,” CIA Director George Tenet told the Senate Armed Services Committee.

Tenet said he is worried that a new audio message attributed to terrorist mastermind Osama bin Laden is a prelude to a strike.

“He’s obviously raising the confidence of his people. He’s obviously exhorting them to do more,” Tenet said. “What he’s said is often followed by an attack.”

Meanwhile, a report surfaced Wednesday of a new audio recording by bin Laden in which the al-Qaida leader purportedly predicts his own death in an unspecified act of “martyrdom” against the United States.

Al-Ansaar, a British-based Islamic news agency, said it believed the 53-minute tape, allegedly recorded earlier this month and acquired from an unidentified man on the Internet, was a carefully worded last will and testament from bin Laden.

U.S. counterterrorism officials said they were reviewing a transcript of the tape but could not verify its authenticity without hearing the actual recording.

Fearing the worst, U.S. officials deployed Avenger anti-aircraft missiles and extra radar around Washington since President Bush’s decision Friday to raise the alert status from yellow to orange, the second-highest level. The Air Force has stepped up its combat air patrols over the capital, defense officials said.

FBI personnel assigned to rapid response teams that would react to any terrorist attacks have been told to have a bag packed for three days’ deployment and put on standby.

U.S. Capitol police were told to carry gas masks at all times. They are in a small, handheld black knapsack about 6 inches long. Every officer has them, including those in plainclothes who provide security for leaders and in the congressional chambers.

The weekly FBI bulletin circulated to 17,000 law enforcement agencies urges police to be extra vigilant for possible chemical, biological and radiological attacks. The bulletin asks local officials to consult a CIA handbook on how to respond to dangerous substances should such an attack occur.

In addition, the National Infrastructure Protection Center, which assesses and warns about threats to critical U.S. infrastructure, issued an advisory describing a heightened chance of global computer hacking as tensions increase with Iraq.