The search for four Navy airmen aboard a jet that disappeared on a training mission over the Pacific was called off after no sign of survivors was found, a Navy spokesman said in San Diego. The missing EA-6B Prowler, a twin-engine aircraft used to jam enemy electronic messages, was reported overdue Monday morning while conducting training exercises about 900 miles off the San Diego coast. “Search and rescue operations to locate the missing crew members terminated at approximately 5:30 p.m.” Monday, said Lt. Gordon Hume, a spokesman for the Pacific Fleet naval air forces in San Diego. “The four air crew (members) were presumed lost at sea. . . . Several pieces of debris sighted . . . were believed to be wreckage from the aircraft, but they sank before they could be recovered.” The missing plane was operating from the aircraft carrier Constellation. Identities of the crew members were withheld.