Is it a signal to militants?

On at least seven occasions in the past six years, a statement by al-Zawahri has been followed by a significant al-Qaida attack within three weeks, an NBC News analysis of the statements show. In four cases, the attacks came within a week.

U.S. officials, speaking on condition of anonymity, say that while they cannot conclusively link all the attacks to the statements, they say the statements have to be taken seriously because of an apparent pattern.

One described U.S. fears that al-Qaida operatives use the statements as possible "go signals" to initiate attacks. This official also noted that al-Zawahri's call to arms that aired on Dec. 19, 2004, was a key factor in the U.S. decision to raise the terrorism threat level two days later.

The four occasions where a Zawahri statement was most closely followed by an attack were:

* On Aug. 6, 1998, Zawahri sent a statement to a London-based Arabic newspaper saying: "We are interested in briefly telling the Americans that their messages have been received and that the response, which we hope they will read carefully, is being readied." The next day, suicide bombers blew up U.S. embassies in Nairobi, Kenya, and Dar Es Salaam, Tanzania, killing more than 220 people. Al-Qaida took responsibility for the attacks.

* On Oct. 9, 2002, a Zawahri tape threatened attacks on the United States, its economy and allies. "I promise you that the Islamic youth are preparing for you what will fill your hearts with horror..." Three days later, bombs destroyed a Bali nightclub leaving more than 200 people, mostly westerners, dead. Officials in the U.S., Australia and Indonesia later said al-Qaida financed the attack.

* In an Oct. 1, 2004, radio address, Zawahri called on Muslims worldwide to help in the Palestinian struggle. Six days later, al-Qaida attacked three Egyptian tourist resorts frequented by Israelis on the Sinai Peninsula, killing 34 people, about half of them Israelis.

* On Nov. 29, 2004, Zawahri issued a video statement promising Americans that Muslims will continue to attack them unless the United States changes its policies against Islam. He said that the U.S. invasion of Baghdad is only the beginning of the American occupation, and warned that it would spread to other countries. On the morning of Dec. 6, armed Jihadists attacked the U.S. consulate in Jeddah, Saudi Arabia, killing eight and wounding 15 others.