Baghdad, Iraq ? Emboldened a day after a successful jailbreak, insurgents laid siege to another prison Wednesday. This time, U.S. troops and a special Iraqi unit thwarted the pre-dawn attack south of Baghdad, overwhelming the gunmen and capturing 50 of them, police said.

Although the raid failed, the insurgents’ ability to put together such large and well-armed bands of fighters underlined concerns about the ability of Iraqi police and military to take over the fight from U.S. troops. Sixty militants participated in the assault, which attempted to free more jailed Sunni insurgents, police said.

The attack on the prison in Madain, 15 miles southeast of Baghdad, began with insurgents firing 10 mortar rounds. They then stormed the facility, which is run by the Interior Ministry, a predominantly Shiite organization and heavily infiltrated by members of various Shiite militias.

Four police officers – including the commander of the special unit – died in a two-hour gunbattle, which was subdued only after American forces arrived. Among the 50 captured, police said, was one Syrian.

The U.S. military did not respond to a request for comment about its role in the counterattack. The raid came a day after 100 Sunni gunmen freed 33 prisoners and wrecked a jail, police station and courthouse in Muqdadiyah, a town northeast of Baghdad near the Iranian border.

Soldiers from the 1/10th Mountain Division's 2nd Battalion, 22nd Infantry Regiment, find a cache of ammunition on the outskirts of Baghdad, Iraq, in this photo released Wednesday. Insurgents who tried to overtake a prison Wednesday south of Iraq were foiled in their attempt, a day after a successful raid on a prison in eastern Iraq.

Madain, the site of Wednesday’s attack, is at the northern tip of Iraq’s Sunni-dominated “Triangle of Death,” a farming region rife with sectarian violence – retaliatory kidnappings and killings in the underground conflict between Sunnis and Shiites.

Police have discovered hundreds of corpses in the past four weeks, victims of religious militants on a rampage of revenge killing. At least 21 more bodies were found Wednesday, including those of 16 Shiite pilgrims discovered on a Baghdad highway, police said. Millions were returning home Wednesday at the conclusion of an important Shiite commemoration in the holy city of Karbala this week.

As U.S. officials step up pressure on Iraqi leaders to form a national unity government quickly, the United States’ top military commander said he had underestimated the extent of Iraqi reluctance to come together.

“I think that I certainly did not understand the depth of fear that was generated by the decades of Saddam’s rule,” said Gen. Peter Pace, chairman of the Joint Chiefs of Staff. “I think a lot of Iraqis have been in the wait-and-see mode longer than I thought they would.”

Pace said one solution was for the Iraqis to do a better job of recruiting more Sunnis into the army and for police forces to balance Shiite domination.

The Bush administration views formation of a broad-based government as a first step in quelling violence and allowing the start of an American troop withdrawal this summer.

While the U.S. military has touted its progress in training the Iraqi army and police, a top expert on Iraq said the forces remained poorly matched against the insurgency and al-Qaida.

“The police have almost no protected vehicles, few heavy weapons similar to those of insurgents, are often located in extremely vulnerable buildings, and have weak communications. Corruption is a major issue,” Anthony H. Cordesman, of the Center for Strategic and International Studies, wrote in a position paper released this week.