An electrical explosion in at a mixed-use condominium complex sent two workers to hospitals on Tuesday. One contractor was critically injured with 40% of his body burned, according to a report from the Westside Today. The other man suffered moderate burns.

The explosion happened in the electrical room in the basement of the Santa Monica, California, complex, which was not open and had no tenants yet. The blast apparently involved an electrical panel, the Associated Press reported.

Firefighters ventilated the structure and prevented anyone from entering until Southern California Edison crews arrived. The cause is under investigation by Southern California Edison and California OSHA.