A U.N. special rapporteur for human rights is calling on the Mexican government to conduct a thorough probe of an army killing of 22 suspected gang members as a possible case of "summary executions."

Christof Heyns applauds the detention of seven soldiers and a lieutenant in the June 30 incident. He urged on Monday that the investigation be swift and that the surviving witnesses be protected.

The army initially reported that the 22 died in a fierce shootout in the town of San Pedro Limon in rural southern Mexico. But The Associated Press reported no evidence of a shootout at the scene and bullet holes indicating some were shot against a wall a close range. A witness later told the AP that 21 were killed after they had surrendered.