The flash fire blew out a lab window at Amgen on Veterans Boulevard Wednesday night.
That blast injured two employees. One suffered severe burns.
Firefighters say the employees were working with ether and other hazardous chemicals.
Another Amgen employee was hospitalized in May 2013 during a flash fire at the building.
Cal-OSHA and the San Mateo County district attorney are working with fire investigators to see what's going wrong.