On Wednesday, sex crimes investigators for the New York Police Department received a troubling report: The influential leader of a Brooklyn safety patrol known as the shomrim had been sexually abusing a teenage girl, the police were told.

A day later, detectives arrested the man, Jacob Daskal, a leader of one faction of what has been, since the 1970s, a sort of auxiliary police force for the ultra-Orthodox Jews of Brooklyn’s Borough Park, Crown Heights, Flatbush and Williamsburg neighborhoods.

Mr. Daskal, 59, was charged with statutory rape, sexual abuse and other crimes. The authorities believe the abuse took place at Mr. Daskal’s home between August and November of last year, when the girl, who is now 16, was a year younger. But the inquiry is continuing, to determine if the alleged abuse occurred over a longer period of time or if there were additional victims.

The revelations cast another shadow over a group that has long cultivated relationships with New York’s law enforcement and elected leaders — and that has secured government funding for vehicles, phones and other equipment integral to its brand of security for some of the city’s most insular populations. On several occasions, critics have questioned whether the shomrim’s proximity to authority has fostered vigilantism or corruption.