After that, the agency sent him to their new office in Denver. One of his first major cases was to track down a brakeman who stole $10,000 from a Wells Fargo express car. He trailed the man to Mexico City. He couldn’t arrest him there, but he kept an eye on him. He kept track of everything he bought with the stolen money. Eventually the man thought the heat was off and headed back to his home in Leavenworth, Kansas via El Paso, Texas. As soon as he reached Leavenworth, Siringo arrested him.
He worked on several smaller cases, the most notable being stopping a gang of rustlers who had used their positions as employees to steal cattle. Another time, he pretended to be an outlaw so he could infiltrate an outlaw hideout to catch up to the crook Bill McCoy. He went to Aspen, Colorado, and disguised himself as a miner to break up a gang of ore thieves in the silver mines.
One case that took a particularly long time to solve started in the small town of Tuscarora, Nevada. Two wealthy mine and mill owners, C. W. Prinz and George Pelling, hired the agency to find out who blew up their homes. Siringo suspected a man named Tim had been “in on it.” He took Tim prospecting near Wichita Falls, Texas, hoping to get a confession out of him. They roamed around northern Texas and Oklahoma for some months before Tim finally told him what he wanted to know. On the way back, Charlie lead him through Denver, where Siringo’s boss