Mexican prosecutors announced homicide charges Tuesday against three soldiers in the June killings of 22 suspected gang members in southern Mexico, challenging the army's version that the suspects died in a shootout.

Attorney General Jesus Murillo Karam said that after a brief initial firefight the three soldiers entered a warehouse where the suspects had taken refuge and opened fire with "no justification whatsoever."

Murillo Karam offered no details on whether the suspects had already surrendered, as a witness has said, or whether they were unarmed when shot.

The attorney general's version raised more questions about what really happened that June 30 in the town of San Pedro Limon, especially how three soldiers could kill 21 suspects without anyone trying to escape. One suspect was killed in the initial firefight, according to the witness.

"Doesn't it seem strange that eight soldiers face off against 22 suspects and all the deaths are on the side of those with numeric superiority?" said Alejandro Hope, a Mexico City-based security analyst. "What was this, a squad of Rambos? Or had the suspects already been disarmed? Whichever way, this doesn't smell good."

The military initially said that all died in a fierce and prolonged gunbattle. Murillo Karam said the firefight lasted eight to 10 minutes.

A witness to the confrontation who asked that her name not be used for fear of reprisals told The Associated Press that the 21 had already given up and walked out of the warehouse and that soldiers led them back inside and killed them.  She also said all of the soldiers at the scene were involved in the shooting, not just three, though she couldn't remember exactly how many soldiers were present.

An AP review of the warehouse days after the shooting showed a few stray shots and no sprays of gunfire within the large grain warehouse where the alleged gang was hiding. Instead, bullet holes and blood stains indicated some were shot near or against the walls at close range.

Photographs from the scene leaked to a local news agency showed most of the bodies against the wall, with guns lying near or propped against them to appear as if they went down shooting.

"If they (the suspects) were poorly armed then you could detain them and kill them, but that only makes the army's argument worse. Because one side was poorly armed and they slaughtered them with no compunction," said Raul Benitez, a security expert at Mexico's National Autonomous University.

Last week, the army detained one officer and seven soldiers for disobedience and dereliction of duty in relation to the incident. Those charges are separate from the attorney general's civil probe.

Murillo Karam said Tuesday a squad of seven soldiers and a lieutenant were involved.

But he said five of the eight-man squad appeared to have stayed outside while the killings occurred. He said there were doubts about one enlisted man's account that could lead to a fourth set of charges.  They would be formally arraigned on Wednesday, he added.

"From the start, we had detected some inconsistencies," he said.

But he did not explain why it took federal investigators 2 1/2 months to file charges, when the inconsistencies emerged days after the incident. His office would not say when it took the case from the prosecutor in Mexico state, where the incident occurred. AP reporters revisited the bodega two weeks ago, and there were no signs of a federal investigation. But photographs in the national newspaper Milenio on Tuesday showed the warehouse roped off with attorney general's office caution tape and markings of the bullet holes, indicating that federal investigators only arrived in the last two weeks.

Nor did Murillo Karam explain why his findings directly contradicted those of Mexico state prosecutors, who said in a July 15 statement that their investigations had found "an equal exchange of fire" and "no evidence at all of any possible execution."

Rights groups have said the killings may prove to be one of the most serious massacres in Mexico in recent memory.

On Monday, a U.N. special rapporteur for human rights called for the Mexican government to conduct a thorough investigation of the incident as a possible case of "summary executions."