WASHINGTON—Enrique Tarrio, the leader of the Proud Boys extremist group, has a past as an informer for federal and local law enforcement, repeatedly working undercover for investigators after he was arrested in 2012, according to a former prosecutor and a transcript of a 2014 federal court proceeding obtained by Reuters. In the Miami hearing, a federal prosecutor, a Federal Bureau of Investigation agent, and Tarrio’s own lawyer described his undercover work and said he had helped authorities prosecute more than a dozen people in various cases involving drugs, gambling, and human smuggling. Tarrio, in an interview with Reuters Tuesday, denied working undercover or cooperating in cases against others. “I don’t know any of this,” he said when asked about the transcript. “I don’t recall any of this.” Law-enforcement officials and the court transcript contradict Tarrio’s denial. In a statement to Reuters, the former federal prosecutor in Tarrio’s case, Vanessa Singh …