Two private security guards on the set of NBC’s Chicago Fire were injured Thursday when youths broke through a barrier and drove a four-wheeler onto the drama’s Chicago set.

The incident occurred in the city’s South Loop near the downtown Fire Academy. The guards, whose names were not released, were ultimately struck by the four-wheeler. A source told Deadline that the heroic efforts by the guards prevented the Atv from reaching the cast and crew.

The youths managed to flee the scene. The guards — retired members of the Chicago Police Department — were hospitalized with non-life threatening injuries.

The incident immediately drew chatter on Twitter on the second to last day of filming for the NBC series. The plan is to wrap Friday.

Good evening everyone: unfortunately there was another serious safety issue on a set today. Details will likely come out soon but there was at least...