Col Chessani is the sixth marine to have charges dropped
A US military judge has dropped charges against a marine officer accused of failing to investigate the 2005 killing of 24 Iraqi civilians in Haditha.
Lt Col Jeffrey Chessani, 44, was the highest-ranked soldier to have been accused of involvement in the incident.
Prosecutors say that US marines indiscriminately killed 24 unarmed Iraqis, including women and children.
Seven of the eight men initially accused have now had all charges dismissed or been acquitted.
The charges against Mr Chessani were thrown out after the judge ruled that a four-star general who oversaw the investigation was improperly influenced by an investigator who later became his adviser.
The judge has indicated that prosecutors can refile the case, but they did not immediately announce an intention to do so.
"We are grateful for the judge's ruling today. He was truly the last sentinel to guard against unlawful command influence," said Richard Thompson, president and chief counsel of the Thomas More Law Center, which represented Mr Chessani.
The case stemmed from a roadside bombing in Haditha on 19 November 2005 which killed one marine, L/Cpl Miguel Terrazas, and injured two others.
The US military at first reported that the Iraqis had been killed by that explosion, 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.
The troops 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, shot by a local human rights activist.
A subsequent investigation by Time magazine suggested that most of the dead were shot by marines after the bomb - and in March 2006 a criminal investigation was begun.
The following month, three officers in charge of troops in Haditha were stripped of their command and reassigned.
Those accused of involvement in the incident have maintained they were reacting to insurgent fire.
Of the eight marines accused, six had all charges against them dropped, and a seventh - Lt Andrew Grayson - was acquitted after facing a court martial.
Only one marine - Sgt Frank Wuterich - still faces charges in relation to the incident.
Sgt Wuterich will be tried on nine counts of voluntary manslaughter later this year.