MEXICO CITY – Joseph Proctor told his girlfriend he was popping out to the convenience store in the quiet Mexican beach town where the couple had just moved, intending to start a new life.

The next morning, the 32-year-old New York native was dead inside his crashed van on a road outside Acapulco. He had multiple bullet wounds. An AR-15 rifle lay in his hands.

His distraught girlfriend, Liliana Gil Vargas, was summoned to police headquarters, where she was told Proctor had died in a gunbattle with an army patrol. They claimed Proctor — whose green van had a for-sale sign and his cell phone number spray-painted on the windows — had attacked the troops. They showed her the gun.

His mother, Donna Proctor, devastated and incredulous, has been fighting through Mexico's secretive military justice system ever since to learn what really happened on the night of Aug. 22.

It took weeks of pressuring U.S. diplomats and congressmen for help, but she finally got an answer, which she shared with The Associated Press.

Three soldiers have been charged with killing her son. Two have been charged with planting the assault rifle in his hands and claiming falsely that he fired first, according to a Mexican Defense Department document sent to her through the U.S. Embassy in Mexico City.

It is at least the third case this year in which soldiers, locked in a brutal battle with drug cartels, have been accused of killing innocent civilians and faking evidence in cover-ups.

Such scandals are driving calls for civilian investigators to take over cases that are almost exclusively handled by military prosecutors and judges who rarely convict one of their own.

"I hate the fact that he died alone and in pain an in such an unjust way," Donna Proctor, a Queens court bailiff, said in a telephone interview with the AP. "I want him to be remembered as a hardworking person. He would never pick up a gun and shoot someone."

President Felipe Calderon has proposed a bill that would require civilian investigations in all torture, disappearance and rape cases against the military. But other abuses, including homicides committed by on-duty soldiers, would mostly remain under military jurisdiction. That would include the Proctor case and two others this year in which soldiers were accused of even more elaborate cover-ups.

The first involved two university students killed in March during a gunbattle between soldiers and cartel suspects that spilled into their campus in the northern city of Monterrey. Mexico's National Human Rights Commission said soldiers destroyed surveillance cameras, planted guns on the two young men and took away their backpacks in an attempt to claim they were gang members. The military admitted the two were students after university officials spoke out.

In that case, military and civilian federal prosecutors are conducting a joint investigation into the killings. The military, however, is in charge of the investigation into the allegation of crime-scene tampering.

In the second case, two brothers aged 5 and 9 were killed in April in their family's car in the northern state of Tamaulipas. The rights commission said in a report that there was no gunbattle and that soldiers fired additional rounds into the family car and planted two vehicles at the scene to make it look like a crossfire incident. The Defense Department stands by its explanation and denies there was a cover-up.

The rights commission, an autonomous government institution, has received more than 4,000 abuse complaints, including torture, rape, killings and forced disappearances, since Calderon deployed tens of thousands of soldiers in December 2006 to destroy drug cartels in their strongholds.

The commission has recommended action in 69 of those cases, and the Defense Department says it is investigating 67.

So far military courts have passed down only one conviction for an abuse committed since Calderon intensified the drug war four years ago: an officer who forced a new subordinate in his unit to drink so much alcohol in a hazing ritual that he died. He was sentenced to four months in prison.