Detectives had drawn a blank on the brutal slayings of Javier Huerta, 20, and Gustavo Olivares-Rivas, 28, who were found bound with zip ties and shot multiple times inside a black Nissan Titan truck near Orlando.
But inside they found the can with the butt inside which they sent away for testing.
Astonishingly it led them to Jose Martinez, 50, who was already in custody for another homicide when the evidence came through.
"If I didn't do the job someone else would have," he said, according to an arrest warrant made public on Wednesday, adding that a hit had been taken out on them because they stole 10 kilos of cocaine "from another guy."
He refused to say who, but said it was time for him to pay for what he had done.
"I've killed over 30 guys," he added.
According to the Orlando Sentinel Martinez told cops that he tricked the men who worked together doing masonry jobs by telling he wanted them to do some work on a house.
Then he pulled out two 9mm handguns and told Rivas to call his wife and get money out of the safe hidden in their back yard.
He eventually recovered $170,000 and a further $40,000 from the truck they were found dead in.
He added that he tortured Rivas and then forced them to tie each other up, before he shot them.
Martinez will now be charged with that murder and detectives will look into other murders based on his claims.