An intercepted message among senior al Qaeda operatives in the last several days raised alarm bells that led to the closing of embassies and consulates Sunday across the Middle East and North Africa, CNN has learned.
CNN has agreed to a request from an Obama administration official not to publish or broadcast additional details because of the sensitivity of the information.
Several U.S. officials also emphasized they have been watching growing threats emerging from Yemen for weeks.
Those threats, combined with the coming end of the month of Ramadan, plus the concern over several major prison breaks in the region, all contributed to the U.S. decision to shut down diplomatic installations.
Officials shuttered 22 U.S. embassies and consulates for the day on Sunday amid fears of an al Qaeda attack. On Sunday afternoon, the State Department said it had extended embassy and consulate closures in 15 of the locations until Friday and added four other posts to the list.
"This is not an indication of a new threat stream," the State Department said, "merely an indication of our commitment to exercise caution and take appropriate steps to protect our employees, including local employees, and visitors to our facilities."
The widespread closure of diplomatic posts is an unprecedented move.
"We're doing what is necessary to protect our people," Rep. Adam Schiff, a Democrat on the House Intelligence Committee, told CNN's State of the Union on Sunday.
The closures stretch across a swath of Africa and the Middle East -- as far west as Mauritania, as far south as Madagascar and as far east as Oman. A U.S. global travel alert is also in place.
As White House and national security officials met to discuss the threat and U.S. military forces in the Middle East were put on a higher state of alert, Interpol warned that al Qaeda has been tied to prison breaks in the region that led to the escape of hundreds of terrorists and other criminals.
It's unclear what locations are targeted by the apparent terror plot, U.S. lawmakers said Sunday.
"I think we know a lot more about the when than the where. And you can tell that from the breadth of the closures across North Africa and the Arabian Peninsula," Schiff said. "But the when was very specific in terms of a Sunday. Obviously, that may continue and the closures may continue. The travel warning is more extensive. But this is not the usual kind of chatter, not the more generalized 'death to the Americans' or 'death to great Satan.' "
CNN national security analyst Fran Fragos Townsend said there could be a strategic reason for shutting down the diplomatic offices.
"Once you take targets away, it buys you additional time to try and disrupt, to identify the cell, the operators in country and the region, and work with your partners in the region to try and, you know, get them in custody or disrupt the plot," she said. "So, some of this operationally is about buying time."
Of particular concern is Yemen, where the government is "on high alert against possible attacks in the days to come," said a senior U.S. security official.
Over the weekend, the security around the U.S. Embassy in Yemen was even tighter than last year, when protesters raided it. At least 12 tanks were stationed within 500 meters of the building.
Britain, France and Germany also closed their embassies in the Yemeni capital, Sanaa, on Sunday and Monday for security reasons. No other embassies are affected, they said.
Western targets under threat
"The threat appears to be much worse than it has (been) in a long time," said a senior U.S. security official in Yemen.
Various Western targets -- not just those tied to the United States -- are under threat, two U.S. officials said.
Three sources said the United States has information that members of al Qaeda in the Arabian Peninsula are in the final stages of planning for an unspecified attack.
One of the sources said such preparations appeared to have increased in recent days, with the approaching end of the Muslim holy month of Ramadan. Sunday is Laylet al-Qadr, or the Night of Power, one of the holiest moments on the Muslim calendar.
Said one U.S. official: "It all leads us to believe something could happen in the near future."
Rep. Michael McCaul, chairman of the House Homeland Security Committee, told CBS' "Face the Nation" Sunday that it was "one of the most specific and credible threats I've seen perhaps since 9/11."
"Because of the specificity, because of where it is coming from, the credibility of it, the level of chatter, it seems to be a fairly large operation," he said. "It's giving the intelligence community quite a bit of pause right now."