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 at least 15 people inside. Authorities have not been able to contact the people trapped inside. The tunnel collapse appeared to damage communication lines.

An industrial tunnel in Los Angeles collapsed partially. None of those rescued had major injuries, authorities said.