Matewan, a movie about organizing coal workers in 1920s West Virginia, included a character by the name of C.E. Lively. Lively was used to infiltrate the union and report from the inside. He was hoping that if things came down to a strike, he could convince people to resort to violence, which would bring down the government and the law to break the strike.
Lively is the one who writes the leter to a Pinkerton-style "detecitve agency" - really just hired thugs in those days. Lively claims that there are communists among the union organizers in order to get the troops to come to town.
I forgot to add, it is the letter that was written to the company