Why Hasn't the U.S. Killed Bin Laden's Wingman Ayman al-Zawahiri?

Five years after he succeeded Osama bin Laden as leader of al Qaeda, Ayman al-Zawahiri is still not dead. Experts who can't agree whether he's a has-been or a mastermind can agree on one thing: he's very good at hiding.

U.S. intelligence officials, who tracked bin Laden to a Pakistani compound and killed him, won't say if they know where Zawahiri is, but admit the 64-year-old Egyptian physician-turned-radical is elusive.

Zawahiri has survived at least four U.S. assassination attempts. But there have been no close calls since 2007, almost a decade ago.

Zawahiri, say U.S. officials, has gone to extremes to escape U.S. targeting. His security guards have reportedly married into local tribes for protection, and he has used "green screen" technology to disguise his living conditions in video messages. According to various U.S. officials, Zawahiri is believed to be in a remote area along the Pakistani-Afghanistan border.

U.S. officials downplay his current importance, saying he's an uncharismatic figure who's been driven deeper and deeper into the wilderness as drones wipe out his henchmen. He's been eclipsed by ISIS, they say, and reduced to jealous sniping at the Islamist upstarts who have taken over parts of two countries.

Said one U.S. counter-terrorism official, "He is a marginalized figure fighting for relevance as head of a decrepit institution."

Some experts, however, say U.S. intelligence exaggerates al Qaeda's decline.