America charged with the Massacre of Thousands?

On December 11, 1981, hundreds of civilians were raped and killed when the El Salvadorian government destroyed a village called El Mozote. The government believed the people of this town were helping a group of rebels in the civil war, and the United States heavily aided El Salvador in their fight against Communism during the Cold War. In an article published in the New York Times in January of 1982, Raymond Bonner quotes Rufina Amaya, one of the only known survivors of the massacre, who says the soldiers were wearing uniforms and using helicpoters, which Salvadorian rebel forces were not known to do. Instead, the witness implies, American troops were sent to El Salvador to commit these atrocities. (When the article was published, the Reagan administration denied that the attack had ever taken place) 

The Atlacatl Battalion, who carried out the attacks, was trained and armed by American soldiers. When a forensic investigation took place in the early 1990s, the Truth Commission identified ammunition and cartridges found at the site as having been manufactured for the United States government at Lake City, Missouri. The cartridges, it was reported, was also found to have been fired from United States-manufactured M-16 rifles. 

It has been proven that America supplied the Atlacatl Battalion with weapons and training, but is America and members of the Reagan administration to be held responsible for authorizing a foreign military to use tactics designed to terrorize the civilian population of El Salvador? Were U.S. personnel actually on-site when the massacre at El Mozote took place, and if so, why did these officials do nothing to stop the killings? Lastly, has the U.S. government sought to cover-up its inolvement in the massacre, and prevent justice from prevailing? 

The main issue here boils down to this: Should Americans be responsible for massacres that take place on foreign soil if they supplied the weapons and training to the soldiers who committed the crime?