No one has yet explained exactly how it happened, but a tank of toxic chlorine gas emptied itself into the Indoor Athletic Building (IAB) Monday morning, sending 34 people to the hospital.

After rescuers gave several differing descriptions of why the gas leaked from a transmission line leading to the IAB pool, University officials announced Tuesday that the incident occurred during a routine tank-changing procedure.

A "slight leak" was detected by the maintenance worker making the change, and before procedures were completed, a release of chloring began," Francis J. Toland, assistant director of athletics, said in Harvard's only official statement on the matter.

Neither Toland nor Buildings and Grounds (B&G) administrators explained how the leak escalated, eventually filling the building and chasing early morning swimmers into Holyoke Street, some gasping for air and doubled over in pain.

One maintenance employee familiar with the building said this week that the worker replacing the tank had told him that "the line was shot" before the leak occurred Monday and that reports of the deterioration to supervisors had not resulted in any action.

Lawrence J. Joyce, director of B&G, said he had not heard of the reports, but doubted the information was true.

The leak began sometime before 10 a.m., and the chlorine gas quickly drifted from the basement tank area up to the pool level and throughout the IAB.

Minutes later, three Cambridge fire companies arrived with police and other rescue experts. Firefighter Anthony Iantosca finally stopped the leak at about 11 a.m., after police had sealed off the area and evacuated adjacent Lowell House.

All of the injured, including students, alumni, University employees and Cambridge firemen, were released from area hospitals by Tuesday evening.