SOACHA, Colombia  Julián Oviedo, a 19-year-old construction worker in this gritty patchwork of slums, told his mother on March 2 that he was going to talk to a man about a job offer. A day later, Mr. Oviedo was shot dead by army troops some 350 miles to the north. He was classified as a subversive and registered as a combat kill.

Colombia’s government, the Bush administration’s top ally in Latin America, has been buffeted by the killings of Mr. Oviedo and dozens of other young, impoverished men and women whose cases have come to light in recent weeks. Some were vagrants, others street vendors or manual laborers. But their fates were often the same: being catalogued as insurgents or criminal gang members and killed by the armed forces.

Prosecutors and human rights researchers are investigating hundreds of such deaths and disappearances, contending that Colombia’s security forces are increasingly murdering civilians and making it look as if they were killed in combat, often by planting weapons by the bodies or dressing them in guerrilla fatigues.

With soldiers under intense pressure in recent years to register combat kills to earn promotions and benefits like time off and extra pay, reports of civilian killings are climbing, prosecutors and researchers say, pointing to a facet of Colombia’s long internal war against leftist insurgencies.