LOS ANGELES (AP) — A portion of a huge industrial tunnel under construction in Los Angeles collapsed Wednesday and up to 15 workers may have been trapped, the Los Angeles Fire Department said.

Thirty-one men working in a massive underground tunnel in Los Angeles were rescued Wednesday night after a portion of the structure collapsed, officials said.

An investigation is underway to determine what led to a tunnel, under construction for the Los Angeles County Sanitation Districts, to partially collapse in Wilmington on Wednesday night, July 9 — prompting the rescue of 31 employees and a halt to work for an undetermined amount of time.