FBI agents arrived at a Marine camp in southern Afghanistan to question its only prisoner — an American who joined the Taliban, officials said Wednesday.

John Walker, 20, of Fairfax, Calif., was found among Taliban fighters held at a fortress in northern Afghanistan after an uprising by the prisoners was put down in late November. U.S. officials have not decided how his case will be handled.

He already has been questioned by the CIA, and Marine Capt. Dave Romley confirmed that other, unspecified agencies have come to see Walker, who is being held in a heavily guarded green metal shipping container.

In Washington, a senior defense official said on condition of anonymity that FBI agents had arrived at the base to question Walker.

Walker has told his interrogators that Al Qaeda was possibly planning an attack using biological weapons sometime before the end of the Muslim holy month of Ramadan, another official in Washington said on condition of anonymity. But U.S. officials said he was likely passing on rumors heard in the Taliban trenches.

"It is hard to imagine how Mr. Walker could be in possession of such specific information or knowledge," White House spokesman Ari Fleischer said, though he added the government "does have concerns and that's why alerts have been made." Ramadan ends around Friday.

The Marines are only feeding and providing security for Walker as well as providing medical treatment for a healing gunshot wound, Romley said.

Meanwhile, the 1,300 Marines on the ground in Afghanistan for the last 18 days have increased their patrols around the southern city of Kandahar, the Taliban's former spiritual capital. A reconnaissance team flew back into the base from interdiction efforts near the city. The heavily laden group did not comment on their mission but said it had been extremely cold while sleeping under the stars near the city.

"The Marines are prepared to support opposition forces in Kandahar as required," Romley said. "Kandahar is becoming more orderly. Police are beginning to show up and watch traffic again."

He also said a small cache of weapons was found near the base by a roving "hunter-killer" team.

The Marines also returned a favor by airdropping 17,200 food packets around the Arghanab River on Tuesday evening, said Marine spokesman Capt. Stewart Upton.

A group of Marines passed through a village in the area about six days ago, seeking a place to cross the river that was shallow enough for a Light Armored Vehicle that had a mechanical problem. The villagers tossed a large number of rocks into the river to create a bridge.

Since Marines patrolling at various times along the river identified the villagers to be in need of such humanitarian assistance, they decided to reward them for the help.