SAN DIEGO (AP) — An apparently accidental explosion rocked a Hilton hotel under construction Monday, injuring 13 workers, five of them critically, authorities said.

Three of the five in critical condition were in a hospital burn unit and eight other victims had serious-to-minor injuries, fire department spokesman Maurice Luque said.

San Diego Deputy Fire-Rescue Chief Perry Peake said the blast occurred in an area of the building containing gas, electric and other utilities and that it appears to have been caused by a mechanical failure.