Mexico's governmental human rights agency said Tuesday it is investigating the deaths of 22 people in a clash with the army that one witness has described as a massacre.

The National Human Rights Commission expects to conclude its report on the incident in about six weeks.

The agency is examining various aspects including reconstructing how the victims died, commission president Raul Plascencia said.

The Mexican army reported on June 30 that 22 presumed criminals had been killed and one soldier wounded in what it described as a shootout after suspects attacked soldiers first.

That version was cast under doubt due to the lopsided death toll and physical evidence at the scene suggesting at least some of those killed had been standing against a wall and shot around chest level.

A woman who says she witnessed the incident told The Associated Press recently that one of the victims died in an initial confrontation and the rest were killed after surrendering — including her 15-year-old daughter, who was already wounded and lying on the floor.

Plascencia said his commission has spoken with the witness twice and she has not asked for protection.

After her account of the killings emerged, international rights watchdogs and the U.S. State Department expressed concern over the incident in the town of San Pedro Limon, in rural southern Mexico, and called on Mexican authorities to investigate.

Mexican President Enrique Pena Nieto said Monday that the federal attorney general's office is also conducting its own probe.