Fire officials are investigating the cause of a fire that caused substantial damage to the offices at the former indoor soccer fields at 10 Reservoir St. Mansfield Fire Chief Neal Boldrighini said firefighters had to cut a lock and force their way into the closed structure when they arrived at the scene on Aug. 2. Despite the delay, however, they were able to contain the fire to the first floor of the two-story building. The 3,000-square-foot building used to house the offices and locker rooms that were attached to an inflatable sports bubble. The bubble was taken down after the facility closed in 2007. Boldrighini said people had gained prior access to the building as there was “quite a bit of vandalism” and “a lot of stuff” inside that did not belong to owner Andrew Gazzolo.