JAKARTA – The search for the black boxes of a crashed Sriwijaya Air jet intensified Monday to boost the investigation into what caused the plane carrying 62 people to nosedive at high velocity into the Java Sea.The Boeing 737-500 jet disappeared minutes after taking off from Jakarta, Indonesia’s capital, during heavy rain on Saturday, and the search so far has yielded plane parts and human remains but no sign of survivors.Authorities have said signals from the boxes containing the cockpit voice and flight data recorders were detected between Lancang and Laki islands in the Thousand Island chain just north of Jakarta's coast.