LISBON, Portugal (AP) — The first investigative report on the deadly derailment of a popular Portugal streetcar said Saturday that a cable connecting the two cabins essentially snapped.

Wednesday’s crash left 16 dead and injured 21 others. Officials called it one of Lisbon’s worst tragedies in recent memory.

The report by the Office for Air and Rail Accident Investigations released Saturday said the two cabins had traveled not more than about 6 meters (20 feet) when they suddenly lost the balancing force provided by the connecting cable, the report said.

The yellow-and-white streetcar is known as Elevador da Gloria and goes up and down a steep downtown hill. The cabin at the top sped downward.

“The cabin’s brakeman immediately applied the pneumatic brake and the hand brake to try to halt the movement,” the report said. “These actions had no effect in stopping or reducing the cabin’s speed, and it continued accelerating down the slope.”

Examining the wreckage at the site, it was “immediately clear that the connecting cable had given way” at the attachment point to the cabin that was at the top of the hill, the report said.

The streetcar, technically called a funicular, is harnessed by steel cables, with the descending car helping with its weight to pull up the other one. It can carry more than 40 people, seated and standing.

The exact number of passengers in each vehicle at the time has not yet been determined, the report said.

The streetcar that crashed had been in use since 1914.

A final report containing the facts, analysis and conclusions regarding the causes of the accident is expected to be published later. If a final report isn’t possible within one year, an interim report will be published.