Karbala, Iraq ? American AC-130 gunships and tanks battled militiamen Friday near shrines in this Shiite holy city, and fighting was heavy in two other towns south of Baghdad. More than 450 Iraqis were released from the notorious Abu Ghraib jail — some emerging with fresh claims of abuse.

Four people were detained in Baghdad in the killing of Nicholas Berg, the 26-year-old American whose videotaped beheading was shown on an al-Qaeda-linked Web site, said Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq. Two of them were released after questioning, he said.

In Karbala, the U.S. military said it killed 18 fighters loyal to cleric Muqtada al-Sadr, who launched an uprising against the American-led coalition in early April and is wanted in the murder of a rival moderate cleric last year. Hospital officials reported 12 deaths, including two Iranian pilgrims. A driver for the Arab television network Al-Jazeera also was killed.

Much of the fighting was near the city’s Imam Hussein and Imam Abbas shrines, which U.S. forces say are being used by militiamen as firing positions or protective cover.

At least six people were killed and 56 were injured in fighting in Najaf and neighboring Kufa, where al-Sadr delivered a defiant sermon to 15,000 worshippers in which he urged his supporters to resist the coalition.

At a checkpoint in Kufa, American forces shot at a car carrying a close aide of al-Sadr, Mohammed al-Tabtabaei, injuring him and killing his driver, al-Sadr’s office in Najaf said. Al-Tabtabaei was taken into custody.

Abu Ghraib releases

Meanwhile, the U.S. military said 454 prisoners were released Friday from the Abu Ghraib prison on the western outskirts of Baghdad. Between 3,000 and 4,000 people are still believed held at the prison. The military is still sending detainees considered security risks to Abu Ghraib.

A convoy of at least six buses, accompanied by U.S. troops in armored vehicles and jeeps, took detainees to several areas, including Tikrit and Baqouba, north of the capital.

Some of those who were freed Friday told of beatings and psychological abuse. They kissed the ground and kneeled to pray after walking out of the police compound in Baqouba.

Abdul Salam Hussain Jassim, 18, said he was held for three months after an explosion.

“Don’t even talk about torture. They destroyed me,” Jassim said. He said a family of five brothers and sisters was detained in the same block and that one of the men was beaten so badly he died two days later.

The Pentagon has begun criminal investigations of at least 37 deaths involving detainees held by U.S. forces in Iraq and Afghanistan, officials said Friday. There are 33 cases involved, the officials said, eight more than the military reported two weeks ago.

The military periodically frees prisoners from Abu Ghraib, which was also notorious as the site of executions and torture during Saddam Hussein’s regime.

The release came as new photographs and shots from a video of alleged abuse and sexual humiliation of Iraqi prisoners were published in The Washington Post. The newspaper reported that some prisoners at Abu Ghraib were ridden like animals, fondled by female soldiers, forced to curse their religion and required to retrieve their food from toilets.

Beheading investigation

Kimmitt said coalition forces on Wednesday captured four people suspected of involvement in Berg’s killing, and were still questioning the two still in custody.

“I don’t know their prior affiliations or prior organizations,” he said. “We have some intelligence that would suggest they have knowledge, perhaps some culpability.”

An Iraqi security official also told The Associated Press that four people were arrested in the case, but he appeared to be referring to a different group of detainees that he said was led by a relative of Saddam.

The official said Iraqi police on May 14 arrested four suspects — all former members of Saddam’s Fedayeen paramilitary organization — in a raid in Salaheddin province, north of Baghdad. The group was led by Yasser al-Sabawi, a Saddam nephew who was not among those captured, the official said on condition of anonymity.

Berg, of West Chester, Pa., was in Iraq seeking business for his communications company. His body was found May 8 near a highway overpass in Baghdad. He was last seen April 10 when he left his Baghdad hotel.

American officials have previously said they believe Abu Musab al-Zarqawi, a Jordanian wanted for allegedly organizing attacks on U.S. troops in Iraq on behalf of al-Qaida, personally carried out Berg’s decapitation.

Aerial attack ‘legitimate’

In Washington, Joint Chiefs of Staff Chairman Gen. Richard Myers said the United States was investigating possible links between al-Zarqawi and alleged foreign fighters who were hit by a U.S. aerial attack Wednesday in Mogr el-Deeb, a desert village near Iraq’s border with Syria.

“We feel at this point very confident that this was a legitimate target, probably foreign fighters, still to be determined their relationship to Zarqawi, but not out of the question,” he said.

But survivors of the attack, which killed up to 45 people, told the AP that the U.S. military had struck a wedding party and that there were no foreign fighters among them. An AP reporter was able to identify at least 10 of the bodies as those of children.