After years in which he became a familiar television presence, al Qaeda's number two leader Ayman al-Zawahiri has "gone dark," according to people who monitor audio and video messages released by the terror group.
Zawahiri has not been seen or heard since December 2009, "the longest gap he has had in nearly six years," according to Ben Venzke of the IntelCenter, which monitors al Qaeda messages for governments, businesses and the media.
A US official said there is "no reason" to believe that Zawahiri has been killed or injured in recent months but that "it is possible" he and other al Qaeda leaders "have new concerns about their security."
Al Qaeda safe havens in North Waziristan in Pakistan have been under a sustained air attack by CIA-directed Predator aircraft, as part of the Obama administration's escalation of efforts against the terror group.
"They may not want to expose themselves through the process of making the tape and sending it out through couriers," the US official said.
Venzke says the disappearance of another frequent al Qaeda speaker, Abu Yahya al-Libi, is the longest gap "since he first began to regularly appear in al Qaeda messaging in 2006."
He has not appeared in an al Qaeda propaganda tape for 137 days, reported the IntelCenter.
Some intelligence analysts believe such absences precede major terror attacks by al Qaeda.
"I don't like it, it's too quiet," said another person who monitors websites for al Qaeda and terror-related messages.
But US intelligence officials say there is "no clear pattern" of messages or their absence prior to recent al Qaeda and Taliban operations.