Jamundi, Colombia ? On a dirt road dotted with country homes near the western city of Cali, three trucks carrying an elite squad of anti-narcotics police pulled up to the gates of a psychiatric center for a planned raid about an hour before dusk.

Within minutes, all 10 officers in the U.S.-trained unit were dead in a ferocious attack that stunned Colombians and severely embarrassed President Alvaro Uribe just as he was savoring a crushing re-election victory.

That’s because the alleged killers were no typical outlaws. The gunmen firing from roadside ditches and from behind bushes were a platoon of 28 soldiers who unleashed a barrage of some 150 bullets and seven grenades, according to a ballistics investigator.

An 11th man, an informant who led the police squad to the scene promising they would find a large stash of cocaine, also was found dead. When investigators removed his ski mask, they found a bullet hole in his head.

In the hours after the May 22 ambush, the head of the army stood by his men, calling the massacre a tragic case of friendly fire, with the soldiers likely having mistaken the armed police for leftist rebels known to operate in the area.

But the nation’s chief criminal investigator quickly produced a more chilling motive.

“This was not a mistake, it was a crime – a deliberate, criminal decision,” chief federal prosecutor general Mario Iguaran told a shocked nation June 1. “The army was doing the bidding of drug traffickers.”

Fifteen soldiers, including the colonel who commanded them, will face charges of aggravated homicide.