Three people were released from care and one remained hospitalized Thursday evening, a day after they were injured in an explosion at Superior Signal Company that killed one employee, Old Bridge police said. Authorities have yet to determine the cause and origin of the explosion that killed one employee and sent a nearby public school into lockdown as authorities doused the subsequent fire. Superior Signal did not immediately respond to messages seeking comment Thursday evening. The Middlesex County Prosecutor’s Office said Wednesday afternoon the initial investigation by state and local autho…