America’s high-security prisons in Iraq have become “terrorist academies” for the most dangerous militant groups, according to former inmates and Iraqi government officials.

Inmates are left largely to run their blocks, which are segregated on sectarian lines. The policy has created a closed world run by Iraq’s worst terrorists and militias, into which detainees with no links to insurgent groups are often thrown.

Inmates from Camp Cropper, the U.S. prison at Baghdad airport, described to The Times seeing Al Qaeda terrorists club to death a man suspected of being an informer. Others dished out retribution with razor wire stolen from the fences.

Captain Phillip Valenti, a U.S. officer responsible for prisons, said he knew of at least three cases of prisoners being murdered by inmates.

“We are very concerned about insurgent efforts to recruit and convert detainees,” he said.

U.S. officials said Friday that they were investigating the suspicious death of another Camp Cropper inmate.

Saad Sultan, the Iraqi Human Rights Ministry’s official for prisons, said, “It looks like a terrorist academy. There’s a huge number of these ‘students’; they study how they can kill. And we protect them, feed them, give them medical care. The Americans have no solution to this problem.”

Vying factions often feed fake tips about their enemies to U.S. forces, meaning just about anyone can end up in jail. While the U.S. military is scrupulous about separating Sunnis from Shiite, they pay less attention to keeping seasoned terrorists apart from people picked up in security sweeps.

Abu Tibeh and his colleagues — four Sunnis and four Shiite — were arrested in November when someone told a U.S. patrol that the party offices they were guarding were being used by a death squad. In fact, the party was a moderate secular faction with close ties to the U.S..

At Camp Cropper they were put in halls of about 85 inmates. The Sunni section was controlled by an imam from an Al Qaeda-affiliated group. The Shiite hall was under the authority of a sheikh from al-Mahdi Army, a fundamentalist militia notorious for its death squads.

“I was terrified,” said Abu Tibeh, a balding, podgy Sunni in his mid-30s. “It was psychological warfare.” His colleague Abu Usama — not their real names — also in the Sunni camp, was so horrified that he suffered a minor heart attack.

Every day Abu Hamza, the Al Qaeda cleric who was in his early 20s, would lecture on the evils of the government and the need for resistance. To their dismay, the new inmates’ own party was often the target of these rants, forcing them to keep their identities secret. They slept in shifts, with two standing guard.

One night, Abu Usama recalled, a group of Al Qaeda enforcers, their faces masked by towels, murdered an inmate. “Six of them came, two guarding the door and four to kill him. One of them hit him on the head with a sock filled with rocks. They beat him to death.”

The leader of the assassins told the cowering prisoners that the man had been an informer, although merely being seen talking to a U.S. guard could count, said Abu Usama. Another suspected informer was clubbed to death in the latrines, he said.

When a new inmate arrived, the takfiris, or Sunni fundamentalists, would move in quickly to recruit him. Abu Usama listened politely to one recruiter — a youth half his age — then tried to avoid him.

In the Shiite camp, their comrade Abu Mustafa, a lean 31-year-old, was faring only slightly better. The imam there was Sayyid Adnan al-Enabi, an al-Mahdi Army commander who ordered the men to join prayer sessions and lectures.

Abu Mustafa refused. “No one should force your religion,” he explained this week. In revenge, the al-Mahdi inmates told the U.S. guards that he was planning to escape. The Americans put him in a metal punishment box 6 feet by 4 feet known as “the coffin,” and kept him there for days.

Both men were freed when the leader of their party intervened with the U.S. military.