The commander of a battalion which is accused of killing 24 people in late 2005 is appearing at the military equivalent of a grand jury.
The hearing will decide whether Lieutenant-Colonel Jeffrey Chessani, 43, will stand trial.
He is accused of dereliction of duty for failing to hold an investigation.
Lt-Col Chessani is also accused of failure to follow an order, after he did not launch an investigation into the killings.
Twenty-four Iraqi civilians, including three women, seven children and several elderly men, died in the incident.
Three other officers are also charged with dereliction of duty and three marines face murder charges.
According to a prosecutor, some of the Iraqis were shot in the head, several at such close range, that their bodies had powder burns.
The victims were described in detail. One was a 66-year-old woman, another, a young woman shot while sitting with her back to a wall, and a teenage girl shot while on a bed.
Another woman is said to have been killed while trying to protect several small children.
Women and children died in the attack in Haditha
The prosecutor added that five young men killed near their car were apparently standing still, possibly with their hands in the air to surrender.
The court was told battalion commanders have a duty to report any possible alleged or suspected crime by their troops.
In this case, no such report was made.
Instead, the commander made only a superficial enquiry after the killings.
A defence lawyer disagreed with the prosecution's interpretation of how the Iraqis were killed, saying the account was based on photographs of the bodies taken by military personnel.