BAGHDAD, Iraq – About 100 masked gunmen stormed a prison near the Iranian border yesterday, cutting phone wires, freeing all the inmates, and leaving behind a scene of devastation and carnage – 20 dead policemen, burned-out cars, and a smoldering jailhouse.

At least 10 attackers were killed in the dawn assault on the Muqdadiyah lockup on the eastern fringe of the Sunni Triangle, police said. The raid showed the mostly Sunni militants can still assemble a large force, capable of operating in the region virtually at will – even though American and Iraqi military officials said last year that the area was no longer an insurgent stronghold.

The insurgency’s strength, spiraling sectarian violence and the stalemate over forming a government in Iraq have led politicians and foreign policy experts to say Iraq is on the brink or per haps in the midst of civil war.

In all, 33 prisoners were freed, including 18 insurgents who were detained Sunday during raids by security forces in the nearby villages of Sansal and Arab, police said. It was the capture of those insurgents that apparently prompted yesterday’s attack. The 15 other inmates were a mix of suspected insurgents and common criminals.

In an Internet posting yesterday night, the military wing of the Mujaheddin Shura Council, a militant Sunni Muslim insurgent group, purportedly claimed it carried out the operation. The posting said the group killed “40 policemen, liberated 33 prisoners, and captured weapons.”

The claim was posted on the Iraqi News Web site. Neither the higher casualty toll among policemen nor the captured weapons could not be independently verified.

The cutting of the telephone lines made it impossible for jailers or security men, who apparently did not have cell phones, to call other police for backup. Residents of the town informed authorities of the situation after hearing the firing.

With the wires cut, the insurgents had 90 minutes to battle their way into the law enforcement compound before police reinforcements showed up from the nearby villages of Wajihiyah and Abu Saida, police said. Muqdadiyah is about 25 miles from the Iranian frontier and 60 miles northeast of Baghdad. By the time the insurgents fled, taking away the bodies of many of their dead compatriots, nearly two dozen cars were shot up and set on fire and the jail was a charred mass of twisted bunk bed frames and smoldering mattresses.

Afterward, American helicopters hovered in the air above the jail. Police said residents fired into the air, but it was not clear if the American aircraft were the target. None was hit.

In other violence yesterday, a roadside bomb killed one policeman and wounded three in Baqouba, 35 miles northeast of Baghdad, authorities said.

An American soldier with the 4th Infantry Division was killed by small-arms fire yesterday while patrolling western Baghdad, the American military reported. At least 2,315 members of the American military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

Also in the capital, gunmen killed an employee of the mayor’s office while he was driving in the Dora neighborhood of south Baghdad.

Police reported discovering eight more blindfolded corpses in west Baghdad,some of them under a highway and showing signs of torture, officials said. In Suwera, 50 miles south of Baghdad, four more corpses were found on the bank of the Tigris River.

The execution-style killings have become an almost daily occurrence in a wave of sectarian violence that has left more than 1,000 Iraqis dead since the bombing last month of a Shiite shrine.