MEXICO CITY (Reuters) - Mexico has detained eight military personnel involved in an alleged shootout in June that killed 22 suspected gang members, the defense ministry said in a statement on Thursday.

The confrontation took place on June 30 in Tlatlaya on the southern fringes of the State of Mexico, which borders Guerrero and Michoacan, states plagued by gang violence. At the time, authorities said the dead - 21 men and one woman - were suspected gang members, who fired on soldiers first.

Last week, Mexico's attorney general said it was investigating the case after an alleged witness to the shootout said in a magazine article that the 22 people were executed after surrendering to the army.

In a separate statement last week, the army said it had every interest in a deep investigation and that it would cooperate completely.

On Thursday, the defense ministry said it had detained 7 soldiers for presumed breach of duty and one official for suspected violation of military discipline, disobedience and breech of duty.

Mexican President Enrique Pena Nieto came to office in December 2012 pledging to quell gang violence that has killed more than 100,000 people since 2007.

Homicides are down in Mexico since he took over but the death toll has risen in some places, like the State of Mexico - Pena Nieto's home state.

(Reporting by Anahi Rama; Writing by Aexandra Alper; Editing by Nick Macfie)