GARDEZ, Afghanistan -- The militants crept up behind Mohammed Akhtiar as he squatted at the spigot to wash his hands before evening prayers at the Guantánamo Bay detention camp.

They shouted "Allahu Akbar" — God is great — as one of them hefted a metal mop squeezer into the air, slammed it into Akhtiar's head and sent thick streams of blood running down his face.

American troops dragged Akhtiar out of his home in Gardez, Afghanistan, in May 2003, flew him to Guantánamo in shackles that July and held him there for more than three years. The tribal leader from eastern Afghanistan belonged to an insurgent group and had taken part in rocket attacks on U.S. forces, American officials said.

Akhtiar was among the more than 770 terrorism suspects who were imprisoned at the U.S. naval base at Guantánamo Bay, Cuba, after the Sept. 11, 2001, terrorist attacks. They are, the Bush administration has said, "the worst of the worst."

The Islamic radicals in Guantánamo's Camp Four who hissed "infidel," spat at Akhtiar and assaulted him, however, knew something his captors didn't: The U.S. government had the wrong guy.

"He was not an enemy of the government, he was a friend of the government," a senior Afghan intelligence officer told McClatchy. Akhtiar was imprisoned at Guantánamo on the basis of false information that local anti-government insurgents fed to U.S. troops, he said.

An eight-month McClatchy investigation in 11 countries on three continents has found that Akhtiar was one of dozens and perhaps hundreds of men whom the U.S. has wrongfully imprisoned in Afghanistan, Cuba and elsewhere on the basis of flimsy or fabricated evidence, old personal scores or bounty payments.

McClatchy interviewed 66 released detainees along with a number of local officials, primarily in Afghanistan, and reviewed available U.S. military tribunal documents and other records.

Most of the 66 were low-level Taliban grunts, innocent Afghan villagers or ordinary criminals, the McClatchy investigation found. At least seven had been working for the U.S.-backed Afghan government and had no ties to militants, according to Afghan local officials.

Of course, Guantánamo also houses Khalid Sheik Mohammed, the alleged mastermind of the Sept. 11 attacks, who along with four other high-profile detainees faces military commission charges. Cases also have been opened against 15 other detainees for assorted offenses, such as attending al Qaeda training camps.

But because Guantánamo was set up under special rules that allowed indefinite detention without charges, it's impossible to know how many of the 770 men who've been held there were terrorists.

The McClatchy investigation concluded, however, that many of the detainees there posed no danger to the United States or its allies and were imprisoned because U.S. officials were fearful of mistakenly letting a militant go free.

McClatchy's interviews are the most ever conducted with former Guantánamo detainees by a U.S. news organization. The issue of detainee backgrounds has previously been reported on by other media outlets, but not as comprehensively.

McClatchy also in many cases did more research than either the U.S. military at Guantánamo, which often relied on secondhand accounts, or the detainees' lawyers, who relied mainly on the detainees' accounts.

The investigation found that although U.S. forces often didn't know whom they were holding or how to obtain credible intelligence from them without enough trained interrogators and skilled linguists, prisoners were beaten and abused by military police, prison guards and intelligence officers.