Baghdad, Iraq ? More than three dozen insurgents launched an audacious strike Saturday evening against the notorious Abu Ghraib prison, wounding 20 U.S. troops and a dozen Iraqi detainees.

The large-scale attack represented a rare direct assault against a well-fortified U.S. position in Iraq. It was also one of the more sophisticated strikes against American troops since coalition forces toppled Iraqi President Saddam Hussein two years ago.

Between 40 and 60 heavily armed men swarmed the prison, detonating two car bombs and peppering the facility with rocket-propelled grenades and small arms and mortar fire.

“First they attacked at one corner to make us think that’s where they were coming from, then they attacked at another corner,” said Lt. Col. Guy Rudisill, military spokesman for detainee affairs. “This was a well-coordinated attack. This is something that we have not seen before.”

At least one insurgent was killed, military officials said. Three U.S. soldiers suffered serious wounds and were evacuated for treatment, Rudisill said.

Prisoners inside the facility suffered mostly shrapnel and bullet wounds in the attack, which occurred about 7 p.m.

About 3,500 suspected insurgents and criminals are held at the massive complex, located on the western outskirts of Baghdad, the capital. The number of detainees has risen sharply in recent months due to more aggressive U.S. and Iraqi raids, Rudisill said.

Last summer, shocking photographs of prisoner abuse and sexual humiliation of Iraqi detainees by U.S. soldiers guarding Abu Ghraib circulated around the world. The resulting scandal led to the prosecution of several soldiers and fueled anti-Americanism throughout the Middle East.

Since U.S. forces took control of the prison, insurgents frequently have lobbed mortar rounds inside but not engaged in the type of head-on assault that took place Saturday. Insurgents typically have preferred to remain hidden, working in small groups or on suicide missions, and attacking from afar with mortar rounds and rockets.

The attack Saturday was one of the largest against U.S. forces anywhere in Iraq since the Jan. 30 Iraqi national elections.

It was unclear whether attackers hoped to free detainees, but, “No prisoners were compromised,” said Lt. Col. Steven Boylon, a military spokesman in Baghdad.