Two workers suffered injuries in an explosion at an Amgen Inc. lab in South San Francisco on Wednesday. A report by the San Jose Mercury News said one person suffered burns to the face and characterized the injuries as “critical,” while the other person had burns on the hands. Both were taken to local hospitals. Cleaning solvents used to sterilize equipment reacted with other chemicals in the lab to cause a flash fire reported about 3:30 p.m., according to a fire marshal. Amgen has confirmed the incident but did not provide any additional details on Thursday. “There were two injuries and those individuals received medical attention,” Amgen said in an email statement. “An investigation has begun to determine the cause of the incident.” Last year, Amgen spent $10.4 billion to purchase Onyx Pharmaceuticals Inc., a cancer drug company in South San Francisco. Shares closed up $3.01, or 2.6 percent, to $118.89 on the Nasdaq.