The Pentagon undertook an investigation yesterday to find out who took and released photographs of terror suspects as they were being transported in heavy restraints aboard a U.S. military plane.

Four photographs of prisoners -- handcuffed, heads covered with black hoods and bound with straps on the floor of a plane -- appeared overnight on the Web site of radio talk show host Art Bell.

"Anonymous mailer sends us photos taken inside a military C-130 transporting POWS," the headline said.

The photos offer the first glimpse into the security measures aboard the airplanes used over the past year to transfer prisoners to prisons in Afghanistan and elsewhere around the world, including the high-security U.S. facility at Guantanamo Bay, Cuba.

It has long been known that prisoners are heavily restrained. And photos of prisoners bound and kneeling after arrival in Cuba early this year created a stir among human and prison rights groups.

The plane in the latest photos was a C-130, said Pentagon spokesman Lt. Col. Dave Lapan. But it has not been determined where it was going or when the photos were taken. Officials believe the photos were not authorized and know their release was unauthorized, he said.

The Air Force and the Central Command are investigating the breach, said Pentagon spokeswoman Victoria Clarke.

"We have very, very tight restrictions on any images of the detainees for security purposes and because we have no interest in potentially holding detainees up for any kind of public ridicule," she told a Pentagon news conference.

This is at least the third time prisoner photos have troubled the Pentagon.

Although the Defense Department has restricted the news media in the kind of photos they can take of prisoners in the war on terrorism, officials have discovered that troops posed for photos with the Taliban fighter John Walker Lindh as he was being handcuffed and while he was wearing a blindfold with an obscenity they had apparently scrawled across it.

The military itself takes photos for documentation, and individual soldiers often take their photos as souvenirs of deployments.

In a court motion, Lindh's lawyers said earlier this year that unofficial photos and videos of Lindh were taken aboard the amphibious assault ship USS Peleliu, where he was confined. Officials said an officer confiscated cameras and film and erased digital images.

Also as part of a court filing, Lindh's lawyers released a picture of him in Afghanistan, blindfolded, strapped to a stretcher and naked. Defense officials have said that while that photo may have been shocking, Lindh was naked as part of the preparation for medical treatment.