(CBS) Abu Zubaydah, once Osama bin Laden's chief of operations and now the most senior member of al Qaeda in U.S. custody, has undergone more than 40 interrogation sessions. But according to U.S. officials, he has given up very little information of value, CBS News National Security Correspondent David Martin reports.

He identified members of al Qaeda but most of them were already killed or captured.

He talked about efforts to build a so-called "dirty bomb" that would spew radioactivity, but the U.S. already knew that from documents and materials discovered in Afghanistan.

He claimed there was a plot to attack financial institutions in the northeast United States, but there's no evidence to corroborate that.

And he simply repeated information from the Pakistani press.

"He's good," one official said of Zubaydah. "He's very good."

And very tough. Another official said Zubaydah was "blowing smoke" and deliberately exaggerating al Qaeda's capabilities.

There are nearly 600 prisoners being held by the U.S. in Afghanistan and at the U.S. Naval base at Guantanamo Bay, Cuba. And Secretary of Defense Donald Rumsfeld acknowledged that members of al Qaeda are tough to crack.

Other officials said no prisoner of any real significance is cooperating. Most of them are sticking to cover stories, like "I came to Afghanistan to teach the Koran."

Only when confronted with evidence collected from al Qaeda hideouts and caves in Afghanistan do they reveal their real reasons.

It has been five months since the first prisoners were taken, and as one military officer put it, "We haven't scratched the surface in getting things out of these folks that we need."