An in-flight explosion "very probably" caused the crash nine days ago of a Mexicana airliner in which 166 persons died, and a bomb may have been aboard, a spokesman for Mexico's main airline pilots' union said today.

In London, two senior officials of the international federation of pilots' unions were quoted as saying that Mexican union representatives had said that there was substantial evidence that a bomb caused the crash.

However, the Mexican union spokesman here denied that his union's delegates in London, who were attending a routine conference there, had told anyone that a bomb caused the crash. The spokesman, Jaime Gonzalez, said that his organization had not reached a formal conclusion about the crash.

Gonzalez, press secretary for the Mexican airline pilot's union, said that union members believed that the scattering of the Boeing 727-200's wreckage over a widespread area indicated that it had blown up in midair. "Very probably, there was an explosion," Gonzalez told reporters at his office. Gonzalez said that the explosion might have been caused by a bomb, but he cautioned that "there are other possibilities as well."

In Washington, a U.S. source familiar with the probe said, "We know for sure there was an in-flight breakup and an in-flight fire, beyond that we don't know." The source conceded, however, that "I can't envision any other flight regime besides a bomb that would cause that." U.S. officials cautioned that although two U.S. observers are in Mexico, so far they have little hard information about the accident.

United Press International quoted Mexican pilots as saying privately that they believed that a bomb had caused the crash, and that someone may have planted it to recover insurance money from one of the victims. Union spokesman Gonzalez said that such a possibility "cannot be discounted" but that "it seems overdramatized."

The pilot union officials' statements fueled speculation here that a bomb had been planted on the plane. Investigators have determined that a fire apparently broke out in the plane's cargo area, so a bomb could have been hidden in luggage.

Mexicana, the government-controlled airline that owned the plane, said Friday that there had been anonymous telephone calls and other messages asserting responsibility for the crash in the name of five separate organizations. Three of the claimants said that they were protesting U.S. or Israeli policy in the Middle East, while the other two cited Central American issues.

The company said, however, that such calls were "common" after airline accidents, and the Interior Ministry said that there was no basis yet for believing that a terrorist group had caused the crash. Both Mexicana and the government have declined to speculate about the crash before a report is released by the official investigating committee, which is expected to take until early May to finish its work.

Experts on aviation disasters noted that the breakup of a plane in flight does not require an explosion or a bomb. The structural capability of an airframe can be exceeded by unusual control commands that might result from an emergency situation. The experts cited the explosion of a tire in a wheel well as one such situation. Catastrophic engine failure is another possible cause.

The plane crashed 90 miles northwest of Mexico City on March 31 en route from the capital to the coastal resort of Puerto Vallarta. There were no survivors in what was the worst plane crash in Mexican history. Nine Americans were among the victims.

Union spokesman Gonzalez said that the plane's tail section was found on one side of a mountain, and the rest of the wreckage was found on the other, indicating that the plane broke up before crashing.

"There were parts of the aircraft that were very far from the rest of the wreckage," Gonzalez said. Witnesses were quoted on the day of the crash as saying that the aircraft burst into flames before crashing.

Reg Smith, president of the International Federation of Air Line Pilots Associations, representing 60,000 pilots from 66 countries, said in a press conference in London that "possibly" the U.S. delegation would put forward a proposal for a pilots' boycott of countries linked to terrorism. He said the U.S. Air Line Pilots Association had polled its members and implied that they were in favor of it.