Two US marines, including a battalion commander, are to face a court martial in connection with the killing of 24 Iraqi civilians in Haditha in 2005.
Lt Col Jeffrey Chessani is the highest-ranking officer charged
Lt Col Jeffrey Chessani is charged with dereliction of duty and failing to report and investigate the deaths.
L/Cpl Stephen Tatum is accused of involuntary manslaughter, reckless endangerment and aggravated assault.
The two men are the first to be sent to court martial in the biggest criminal case involving civilian deaths in Iraq.
Prosecutors allege that the marines shot unarmed people in retaliation for a roadside bomb attack that killed one of their comrades.
In a ruling released at Camp Pendleton, California, Lt Gen James Mattis said he had decided to refer the charges against the two men to a general court martial after reviewing evidence presented during Article 32 investigation hearings.
Lt Col Chessani is the most senior US serviceman since the Vietnam War to face a court martial for actions in combat.
L/Cpl Tatum was initially charged with unpremeditated murder
Eight US soldiers were originally charged in connection with the killings, which sparked international outrage.
In August, murder charges against one of L/Cpl Tatum's fellow marines, L/Cpl Justin Sharratt, were dropped by military prosecutors because they were not supported by sufficient evidence.
Charges against another murder suspect, Sgt Sanick Dela Cruz, were dropped in April in exchange for his testimony.
Two weeks ago, investigators recommended withdrawing murder charges against Sgt Frank Wuterich, who is accused of leading the massacre, and instead trying him for negligent homicide.
Four officers were initially charged with dereliction and failing to report and investigate the killings. Those against Capt Randy Stone and Capt Lucas M McConnell have been dismissed, whereas 1st Lt Andrew A Grayson is awaiting a preliminary hearing.
Twenty-four Iraqi civilians, including three women, seven children and several elderly men, died at Haditha, in Anbar province, on 19 November 2005.
Lt Col Jeffrey R Chessani: Violation of a lawful order, dereliction
1st Lt Andrew A Grayson: Dereliction, false official statement, obstructing justice
Capt Lucas M McConnell: Dereliction; Charges dismissed
L/Cpl Stephen B Tatum: Involuntary manslaughter, reckless endangerment, aggravated assault
Staff Sgt Frank D Wuterich: Unpremeditated murder, soliciting another to commit an offence, false official statement
Sgt Sanick P Dela Cruz: Unpremeditated murder, false official statement; Charges dismissed in exchange for testimony
L/Cpl Justin L Sharratt: Unpremeditated murder; Charges dismissed
Capt Randy W Stone: Violation of a lawful order; dereliction of duty; Charges dismissed
The US military at first reported that the Iraqis had been killed by the improvised explosive device (IED) that killed L/Cpl Miguel Terrazas, or in a subsequent gunfight with insurgents.
But Iraqi witnesses said the US troops shot dead five unarmed men in a car when they approached the scene of the bombing in a taxi.
They were then accused of killing 19 other civilians in three houses nearby over the next few hours.
Despite the accusations, there was no full US investigation into what happened until January 2006, when video footage emerged of the aftermath taken by a local human rights activist.
After a report in Time magazine showed flaws in the initial marine statement, a preliminary investigation was begun. The inquiry confirmed civilians had been shot in their homes, but described the deaths as "collateral damage".
In December, military authorities charged four marines with unpremeditated murder and another four with failing to properly report or investigate the deaths.