KABUL, Afghanistan -- American soldiers herded the detainees into holding pens of razor-sharp concertina wire, the kind that's used to corral livestock.

The guards kicked, kneed and punched many of the men until they collapsed in pain. U.S. troops shackled and dragged other detainees to small isolation rooms, then hung them by their wrists from chains dangling from the wire mesh ceiling.

Former guards and detainees whom McClatchy interviewed said Bagram was a center of systematic brutality for at least 20 months, starting in late 2001. Yet the soldiers responsible have escaped serious punishment.

The public outcry in the United States and abroad has focused on detainee abuse at the U.S. naval base in Guantánamo Bay, Cuba, and at the Abu Ghraib prison in Iraq, but sadistic violence first appeared at Bagram, north of Kabul, and at a similar U.S. internment camp at Kandahar Airfield in southern Afghanistan.

"I was punched and kicked at Bagram. . . . At Bagram, when they took a man to interrogation at night, the next morning we would see him brought out on a stretcher looking almost dead," said Aminullah, an Afghan who was held there for a little more than three months. "But at Guantánamo, there were rules, there was law."

Nazar Chaman Gul, an Afghan who was held at Bagram for more than three months in 2003, said he was beaten about every five days. American soldiers would walk into the pen where he slept on the floor and ram their combat boots into his back and stomach, Gul said. "Two or three of them would come in suddenly, tie my hands and beat me," he said.

When the kicking started, Gul said, he'd cry out, "I am not a terrorist," then beg God for mercy. Mercy was slow in coming. He was shipped to Guantánamo around the late summer of 2003 and imprisoned there for more than three years.

According to Afghan officials and a review of his case, Gul wasn't a member of al Qaeda or of the extremist Taliban regime that ran Afghanistan from 1996 to 2001. At the time he was detained, he was working as a fuel depot guard for the U.S.-backed Afghan government.

When U.S. soldiers raided the house he was visiting, acting on a tip from a tribal rival who was seeking revenge against another man, they apparently confused Gul with a militant with a similar name — who was also imprisoned at Guantánamo, according to an Afghan intelligence official and Gul's American lawyer.

The eight-month McClatchy investigation found a pattern of abuse that continued for years. The abuse of detainees at Bagram has been reported by U.S. media organizations, in particular The New York Times, which broke several developments in the story. But the extent of the mistreatment, and that it eclipsed the alleged abuse at Guantánamo, hasn't previously been revealed.

Guards said they routinely beat their prisoners to retaliate for al Qaeda's 9-11 attacks, unaware that the vast majority of the detainees had little or no connection to al Qaeda.

Former detainees at Bagram and Kandahar said they were beaten regularly. Of the 41 former Bagram detainees whom McClatchy interviewed, 28 said that guards or interrogators had assaulted them. Only eight of those men said they were beaten at Guantánamo Bay.

Because President Bush loosened or eliminated the rules governing the treatment of so-called enemy combatants, however, few U.S. troops have been disciplined under the Uniform Code of Military Justice, and no serious punishments have been administered, even in the cases of two detainees who died after American guards beat them.