This article in the Wash Post has the most info I could find:

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The American soldiers were not moving in a convoy at the time of the attack, but rather parked in two Humvees in an area 12 miles west of Mahmudiyah, attempting to prevent insurgents from laying down roadside bombs, said a U.S. military official who spoke on condition of anonymity. Americans frequently conduct such missions, known as "overwatch," to monitor suspected trouble spots at night. It is unusual to have American soldiers out on patrol in as few as two vehicles.

"This was not a convoy, they weren't roving, they were in a fixed location," the official said. "Their mission was to counter the emplacement of improvised explosive devices."

The coordinated attack began when a roadside bomb blew up on the soldiers, followed by gunfire, officials said. The two vehicles went up in flames and were spotted 15 minutes later by a surveillance drone, after a nearby unit that heard explosions could not make contact with the Humvees. The extent of the damage made it difficult to identify the slain soldiers.

On Sunday, Maj. Gen. William B. Caldwell, the top American military spokesman in Iraq, said the interpreter and four soldiers had been killed. Military officials have so far been able to identify all but one of the soldiers, Caldwell said ...