The explosion that destroyed the Israeli Army headquarters in Tyre, Lebanon, on Nov. 11, killing 89 people, was an accident caused by a gas leak, an Army investigating commission reported today.

The commission, headed by reserve Army Maj. Gen. Meir Zorea, said the explosion apparently occurred on the northwest side of the first floor of the seven-story structure where gas had escaped into an open space.

"The explosion caused a significant horizontal movement of the structure and, as a result, its collapse and complete destruction," the commission said in a report to the Cabinet.

Reuter news service reported that the committee also found that the building was constructed without proper standards and supervision. Plans and foundations were made for a four-story structure, and the building had not been strengthened when another three stories were added.

The final confirmed death toll from the incident was 89 persons -- 75 Israeli soldiers and border police guards and 14 Palestinian and Lebanese Arabs.

Shortly after the explosion, the military command here announced that it was caused by a car bomb and there was considerable internal criticism in Israel of the Army's security measures for its own troops in Lebanon.

But the commission reported that it had "completely eliminated" the possibility that the blast was caused by "explosive materials." It "did not encounter one fact which could point to the explosion having been caused by terrorist activity."

The commission recommended that there be a continuing investigation of the building's gas system to determine exactly how the leak and subsequent explosion occurred. It said a major factor that led to the collapse of the structure after the explosion was that "the shell of the building was not planned nor in fact built to stand up under significant horizontal load."