When the witness refused to sign a false statement that 22 suspected drug gang members had died in a shootout with the Mexican army, state investigators began to kick her in the ribs, she said. They put a bag over her head, plunged her face into a toilet bowl and beat her so hard that, six months later, she still has trouble with her hearing and eyesight.

“As they were hitting me … they told me they could make even the mute talk,” said the woman, one of three witnesses known to have survived a June 30 mass killing by the Mexican army.