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 collapse was reported just before 8 p.m. near the intersection of South Figueroa Street and Q Street. As many as 31 tunnel workers were rescued and accounted for following the collapse, according to the officials.