What federal investigators say began as a planned gun sale in Worcester ended with an alleged armed robbery, a foot chase and the arrest of an 18-year-old now facing federal charges.
Joshua Guzman, 18, of Worcester, is accused of assaulting a federal confidential informant during a firearm transaction arranged through WhatsApp messages earlier this year, according to federal prosecutors.
Guzman appeared in federal court in Worcester and remains detained, authorities said…