Well, the US military says that the Marines were out on foot conducting operations outside Hadithah.

NPR: U.S. Envoy Describes Troop Withdrawal

He's embedded with an Army unit along the Euphrates River not far from the Marines in Hadithah.

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Brigadier General CARTER HAM: This morning at about 0630 local time in Iraq, a mounted US Marine element operation near Hadithah was attacked by an explosive device.

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In fact, only last week two American Marines from the same combat team as the six who died yesterday were attacked and killed and that was part of a firefight in the same area near Hadithah--in a village near Hadithah in which there was an air strike eventually, and the US military said it killed nine insurgents.

NPR: U.S. Envoy Describes Troop Withdrawal