The announcement came after a woman said she saw soldiers shoot and kill her 15-year-old daughter, Erika Gomez Gonzalez, in June in the incident even though the girl was lying wounded on the ground.
The mother, who asked not to be identified, said 20 others were shot dead in the town of San Pedro Limon after they surrendered and were disarmed. She said one youth was killed earlier in a gun battle with the troops.
The Mexican government has maintained that all 22 died during a shoot-out at a warehouse when soldiers were fired on in the early morning of 30 June.
That version of events came into question because only one government soldier was wounded, and physical evidence at the scene pointed toward more selective killings.
“We have an investigation and are discussing the case to find out what really happened,” Human Rights Commission president Raul Plascencia said yesterday.
Several days after the killings, witnesses took pictures of the warehouse and found little evidence of sustained fighting. There were few stray bullet marks and no shell casings. At least five spots along the warehouse’s inside walls showed the same pattern. One or two closely placed bullet pocks, surrounded by blood, gave the appearance that some of those killed had been standing against the wall and shot at about chest level.
However, the state of Mexico prosecutors’ office released a statement saying there was “no evidence at all of possible executions”. It said it found ballistic evidence of “crossfire with a proportionate interchange of gunshots”.
The state government refused to release autopsy reports requested under Mexico’s freedom of information law, declaring them state secrets to be guarded for nine years.
Relatives of three other gang members who were killed and a doctor who saw Erika’s body said the wounds were consistent with the mother’s account of how she and the others were killed – an incapacitating wound and a burst of shots to the chest.
The attorney general’s office said its investigation was ongoing but no corroborative evidence had been found.