By JUAN FORERO
 
BOGOTÁ, Colombia, Sept. 7 - The attorney general's office said late Monday that Colombian soldiers assassinated three union leaders last month, an account that contrasts sharply with the army's earlier contention that the three men were Marxist rebels killed in a firefight.

The attorney general's human rights unit on Monday ordered the arrest of an army officer, two soldiers and a civilian who took part in the killings of Jorge Eduardo Prieto, Leonel Goyeneche and Héctor Alirio Martínez on Aug. 5 in Saravena, a town long besieged by leftist rebels. Since 2002, American military trainers have been instructing Colombian soldiers there in counterguerrilla techniques, though it is unclear if the Americans trained the unit accused of killing the union leaders.

"The evidence shows that a homicide was committed," Luis Alberto Santana, the deputy attorney general, said at a news conference on Monday. "We have ruled out that there was combat."

The attorney general's announcement vindicated union leaders in Colombia and Europe who said the army had killed three defenseless union activists and then tried to cover the matter up.

"It's clear that we were never wrong, saying that they were assassinated by members of the Colombian Army," said Domingo Tovar, who coordinates human rights activities for the Central Workers Union, largest Colombian labor confederation.

The attorney general's announcement came days after Secretary of State Colin L. Powell warned the Colombian government that it must curtail rights abuses or risk losing aid. On Tuesday, Vice President Francisco Santos acknowledged that the government had erred in its initial characterization of the killings, saying, "Yes, we were wrong."

Colombia is by far the world's most dangerous country for union members, with 94 killed last year and 47 slain by Aug. 25 this year, according to the National Union School, a research and educational center in Medellín. Most of those killings were by right-wing paramilitary leaders linked to rogue army units. Worldwide, 123 union members were slain last year, according to the International Confederation of Free Trade Unions, a Brussels-based group.

The number of killings of union members has dropped in Colombia, from a high of 222 in 1996. But union leaders, foreign diplomats and political analysts say the government has done little to improve safety - underscored by the fact that the union leaders killed in Saravena had asked the government for better security.

Paramilitary organizations, which use death squads to erode support for rebel groups, have accused unions of working with guerrillas. Rebel groups have, to be sure, drawn some members from unions.

But union leaders have also made enemies of powerful forces in Colombia's highly stratified society, both for their leftist declarations and their harsh criticism of fiscally conservative governments bent on privatizing industries and holding down labor costs.