Colombian officials have arrested eight soldiers wanted in connection with last month's killing of 10 undercover police officers and a civilian in the south.
Colombia's fight against drugs involves different security forces
The authorities first said it was an accidental shooting, with the soldiers mistaking the police for traffickers.
But officials say preliminary inquires suggest links between the troops and drugs gangs operating in the region.
President Alvaro Uribe, who at the time was campaigning for re-election, had urged a speedy investigation.
The Colombian police commandos were working on a drugs operation near the southern town of Jamundi when they were shot.
They came under fire from a group of some 28 soldiers, in an incident initially labelled as a "friendly fire" shooting.
The authorities said there had been a breakdown in communication between the security forces.
President Uribe described the shooting as "extremely serious".
He called on investigators to "quickly tell the country the whole truth so this doesn't turn into another Guaitarilla".
He was referring to a shootout in southern Colombia in March 2004 when seven police officers and four civilians were shot dead by soldiers.
The authorities were criticised for the slow pace of the investigations amid accusations of cover-ups by both the police and the military.
The town of Jamundi is in the Valle region, south-west of the capital, Bogota.
Drug traffickers and the Revolutionary Armed Forces of Colombia (Farc), the country's largest rebel group, are active in the region.