Agents from the Naval Criminal Investigative Service have completed their initial inquiry into the incident last November.
Further investigations may be ordered after Marine Corps and Navy prosecutors review the evidence and determine whether to recommend criminal charges, according to Pentagon officials who discussed the matter on condition of anonymity.
The decision on whether to bring criminal charges will be made by the commander of the Marines' unit.
The Marines reported after the incident on November 19 that 15 Iraqi civilians had been killed by a makeshift roadside bomb and in crossfire during a battle between Marines and insurgents.
In March, Time magazine published a report based on accounts from survivors and human rights groups suggesting that the killings were deliberate acts by the Marines against unarmed civilians including woman and children.
A criminal investigation was then ordered by the top Marine commander in Iraq, Major General Richard Zilmer.
A second investigation is looking into whether officers in the Marines' chain of command tried to cover up the events.
That inquiry, which has not yet been made public, criticises some officers for failing to pursue obvious discrepancies in the initial reports about what happened in Haditha.