Intercepted communication between Al Qaeda leaders was one component of a broader pool of intelligence that prompted a threat alert closing numerous US embassies in the Middle East and Africa, US sources said on Monday.
The New York Times reported that the closure of the embassies was the result of intercepted electronic communications between Ayman Al Zawahri, who replaced Osama bin Laden as head of Al Qaeda, and Nasser Al Wuhayshi, the head of Yemen-based affiliate Al Qaeda in the Arabian Peninsula (AQAP).
US sources said that while some type of message between Zawahri and AQAP was intercepted recently, there were also other streams of intelligence that contributed to the security alert, which was prompted by a threat from AQAP.
"The threat picture is based on a broad range of reporting, there is no smoking gun in this threat picture," a US official told Reuters on condition of anonymity.
US officials said there was still no information about a specific target or location of a potential attack, but the threat to Western interests had not diminished.
The threat is just as serious now as it was on Friday when the State Department issued a worldwide travel alert, said Representative Dutch Ruppersberger, the top Democrat on the House of Representatives Intelligence Committee.
"It's a very serious threat," Ruppersberger told CNN. "I've seen the intelligence. It's a threat coming from the highest levels of Al Qaeda. And especially focused in the Arabian peninsula, Yemen and areas like that."
ABC News on Monday reported that a senior US official said authorities "are frantically searching" for vehicle bombs that Al Qaeda wants to use to blow up the US embassy in Yemen and possibly other embassies.
Yemen's embassy in Washington issued a statement listing 25 "most wanted terrorists" it said were planning to carry out operations in its capital Sanaa, and said it was offering a reward for information leading to their capture.
"The Yemeni government has taken all necessary precautions to secure diplomatic facilities, vital installations and strategic assets," the statement said.
Paul Pillar, a former senior CIA analyst, said it would have taken more than a piece of communication between Al Qaeda leaders for the United States to take such aggressive action as closing 19 US diplomatic posts for a week.
"The fact that Zawahri would be sending such messages doesn't surprise me," Pillar said.
"Standard procedure for analysts is not to bet the farm on any one piece of intel, even if we have no doubt that this is a message from Zawahri. It's a matter of piecing together the implications of different reports, and in combination reaching judgments about the level of threat."
US sources and analysts cautioned that communication between al Qaeda and its affiliate did not necessarily mean that AQAP was taking orders from Zawahri.
"It shows al Qaeda core still runs a global terror machine. It is a complex picture," said Bruce Riedel, a former CIA officer who now directs the Brookings Intelligence Project in Washington.
"Zawahri does not always get what he seeks but is still undisputed as bin Laden's heir," he said.
Daniel Benjamin, a former State Department counterterrorism official who is now at Dartmouth College, said the Al Qaeda affiliate in Yemen is a "worrisome threat" because it has "talented bombmakers" and likes to move quickly and does not spend as much time planning attacks as the broader al Qaeda did.
White House spokesman Jay Carney said while core Al Qaeda had been diminished, affiliate organisations, in particular Al Qaeda in the Arabian Peninsula, had strengthened. "And we have here in Washington identified AQAP as a particularly dangerous threat for some time now," he said.