The US soldier suspected of killing 16 Afghan villagers had been drinking alcohol and was suffering from stress and tensions with his wife on the night of the massacre, it is claimed.
The New York Times quoted a senior American official as saying: “When it all comes out, it will be a combination of stress, alcohol and domestic issues — he just snapped.”
The man, believed to be a 38-year-old father-of-two, was understood to be on his fourth combat tour.
The source, who was speaking on condition of anonymity because the solider has not yet been formally charged, added: “There will be questions raised about his emotional and mental stability for a fourth deployment.”
The soldier’s lawyer, John Henry Browne, told Reuters his client had been wounded twice in three tours in Iraq and had been told he would not be sent back to a war zone.
Speaking at a news conference in Seattle, Browne said: “He and his family were told that his tours in the Middle East were over. His family was counting on him not being redeployed.
"Literally overnight that changed. So I think it would be fair to say that he and the family were not happy that he was going back."
When asked about the New York Times report, Browne acknowledged stress was a factor but dismissed the domestic issue as “nonsense”.
He added he did not know whether the solider had been drinking.
The incident occurred on Sunday in the village of Belandi in the Panjway district of Kandahar province, in the south of the country.
The soldier was detained and has been has been flown to Kuwait.
Although Afghan officials wanted the man to be tried in their country, US officials at the Pentagon said there were not "appropriate facilities" for holding him there. The decision has sparked anger in Afghanistan, where people wanted to see the soldier face a public trial.
Mohammad Naeem Lalai Hamidzai, a member of parliament in Afghanistan and part of the commission investigating the incident told the Associated Press: "It was the demand of the families of the martyrs of this incident, the people of Kandahar and the people of Afghanistan to try him publicly in Afghanistan."
Nine children and three women were among those killed in the shooting rampage, shot as they slept in their beds. Four men were also killed and five others were wounded.
There are differing accounts of the exact circumstances of the shooting, with some Afghan witnesses and officials saying that there was more than one gunman, according to The Telegraph.
Afghanistan's President Hamid Karzai described the attacks as "impossible to forgive", saying in a statement on Sunday: "When Afghan people are killed deliberately by US forces this action is murder and terror and an unforgivable action."
There are concerns that the soldier's removal from Afghanistan could cause further clashes in the country. Mass protests followed the burning of the Koran by two American soldiers at the end of last month. Some analysts viewed the book-burning as a "tipping point" for the country, likely to substantially increase support for the Taliban.
It is likely that the accused soldier will stand trial at a US military tribunal. Leon Panetta, US defence secretary has said that the soldier could face the death penalty.
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