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.

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.