Evan Squires, 70, and his wife Wilda Squires, 65, of Minnedosa were snowbirds who regularly spent the winter months in Weslaco, Texas.
They were shot and killed in their mobile home in the Magic Valley Trailer Park on Nov. 13, 1988.
A jury found Arturo Almaguer (ah-MONG'-gehr), 44, guilty of capital murder.
Assistant district attorney Joaquin Zamora says Almaguer is to be sentenced Tuesday.
Zamora said last week that his office would not seek the death penalty if Almaguer were convicted.
Evan Squires was shot once and his wife twice, said Zamora. It appears the two had been sleeping before the shooting because they were wearing pyjamas.
Zamora had said it was hard to put the case together since many witnesses, some seniors who also lived in the trailer park, have died.
Police who later examined the trailer made an odd discovery: feces on the floor. Experts were able to collect enough skin and blood cells from the feces to get a DNA profile.
Authorities said they got a match when Almaguer was arrested on an unrelated offence in Michigan.