An off-duty police officer involved in a Brewerytown melee in May that left a 14-year-old boy injured has resigned from the Philadelphia Police Department, officials said Wednesday.

Kevin Furman, who had worked for the department since 2013, resigned June 24, police spokesman Jeff Chrusch said.

Furman is the subject of an Internal Affairs probe into a fight that erupted outside a gym May 11, but Police Commissioner Richard Ross said Wednesday that Furman's resignation was a surprise to him and the department. Furman's resignation could be related to the incident but was not prompted by the department, Ross said.

"I shouldn't say it has nothing to do with it, but it has nothing to do with our investigation. To date, even though Internal Affairs is looking at it, his resignation came as a surprise to us," Ross said. "That should tell you right there that there was no conversation about, 'You should consider this.' "

Furman could not be reached for comment. His resignation was first reported by WHYY Newsworks.

On May 11, he was working out at Steelworks Strength Systems gym, 2511 W. Girard Ave., when a group of boys cut through the gym. According to Kobie Xavier, a trainer working at the gym that day, the boys were told to leave. Later they threw rocks at members, including Furman, who were jogging outside.

Xavier said a scuffle ensued in which other people from the neighborhood showed up, some with bats. According to a police report, Naji Tribble, 14, threw a punch at Furman's wife, who was also a member of the gym. Furman then threw the boy to the ground, the report said.

Following the incident, Tribble was charged with simple assault and making terroristic threats, according to the family's attorney, Kevin Mincey. Tribble spent a week in the hospital and is still recovering from head trauma, Mincey said. His family disputes the narrative in the police report.

The Internal Affairs investigation into Furman is ongoing.