Spanish, Dutch and U.S. investigators began to examine tape recordings of the conversations of the Pan Am and KLM Royal Dutch Airlines flight crews yesterday in an attempt to find why the world's worst aviation accident happened.

The recordings - contained in crash-resistant metal cases - were displayed for the press yesterday in the offices of the National Transportation Safety Board, where the technical examination will be done.

The recordings are regarded as critical to understanding how KLM and Pan Am Boeing 747s came to collide March 27 on a runway at Los Rodeos Airport on Tenerife, in the Canary Islands. The death toll was 577, with 67 survivors.

In Dover. Del., today, a team of forensic pathologists, dentists, blood specialists, fingerprint experts and others began work on identifying the bodies of 325 people who were passengers on the Pan Am flight. Those bodies, embalmed on Tenerife Island by Spanish authorities, were flown to Dover Sunday.

The identification work is being done at the mortuary at Dover Air Force Base, the East Coast facility for the armed forces that handled Vietnam War dead.

Officials at Dover yesterday would not comment to reporters on their progress. William H. Waltrip, Pan Am's vice president in charge of operations, told the Associated Press yesterday it could take a week to 10 days to complete identification.

The disaster occurred when the KLM jet struck the Pan Am jet on the runway, KLM was taking off and had reached a speed experts estimate at about 160 to 175 m.p.h. when the collision occurred. The Pan Am jet was taxiing on the runway.

Dutch officials have stated that the KLM captain took off without an official air traffic clearance but that the Pan Am captain had not departed the runway at the exit designated by air traffic control. Pan Am officials insist their pilot was acting properly.

Experts are hoping the cockpits tapes will tell them why the Dutch captain - an experienced pilot on an airline with an outstanding safety record - would take off without approval. The Dutch pilot died in the crash.

Early speculation has centered around a suggestion that radio transmissions from the control tower to the airplanes could have been partially blocked and thus misunderstood.