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.

The Los Angeles Fire Department rushed to Wilmington after a tunnel collapsed and trapped 27 people inside. Four other workers went in after the collapse to help the others. The construction workers inside the underground tunnel were rescued roughly an hour after the collapse,