Major General Richard Zilmer, the commander of the US forces in western Iraq, asked the Navy Criminal Investigative Service to investigate after Iraqis raised the incident with marine leaders on May 1.
"A preliminary investigation conducted by MNF-W (Multi-National Force-West) found sufficient information existed to recommend a criminal investigation into the incident," a US military said in a statement.
It said several marines from the 5th Marine Regiment's 3rd Battalion were suspected of involvement in the incident and have been sent back to the US pending the outcome of the investigation.
The statement provided no details on the incident except to say that it occurred April 26 in the area of Hamanidiya, west of Baghdad.
It is the latest in a flurry of probes into killings of civilians by US troops.
The most serious incident to date are allegations that marines shot and killed at least 15 civilians in the western town of Haditha in November and then claimed they were killed in a roadside explosion.
Representative John Murtha, a Democrat, charged recently that about 24 civilians, not 15 as reported, were killed in "cold blood" at Haditha.