The FBI discounted the possibility that a bomb exploded aboard the Pacific Southwest Airlines jetliner that crashed in San Luis Obispo County last week, killing all 43 aboard. The possibility had been suggested earlier by a National Transportation Safety Board investigator after documents from the airliner were found in a seven-mile swath. "We have found no evidence of any type of explosion," said FBI spokesman Jim Neilson. Authorities believe that fired airline employee David A. Burke, a passenger, may have shot and killed his former supervisor, also a passenger, and then invaded the cockpit, causing the crash. Agents who searched Burke's home found no explosive devices, papers filed in federal court indicated.