Investigators have found that three out of four people in a Hydro One helicopter that crashed in the woods in Tweed, Ont. on Dec. 14 fell to their deaths as the helicopter went down.

The aircraft was downed because of damage to its tail rotor, officials said Thursday.

Transportation Safety Board (TSB) investigator Peter Rowntree said that about 500 metres east of where the helicopter went down, in a wooded area near a horse farm, they found a white canvas bag and metal carabiners attached to it in the snow.

The items were attached to the outside of the chopper and used by hydro line workers to carry their equipment to and from job sites.

The bag and carabiners appeared to have come loose from the outside of the chopper as it approached a landing area and flew back and struck the tail rotor, severely impacting the pilot’s ability to keep the chopper stable, Rowntree said.

“When we lose a piece of that rotor that causes a massive vibration in the helicopter,” he told reporters.

Rowntree said they are still working to determine whether the bag that slipped off the helicopter was improperly secured or whether the carabiners used to suspend it failed.

The pilot lost control of the helicopter and Rowntree said the three passengers in the chopper fell out of the aircraft at an unknown height to their deaths.

“At this point we know this is causal,” Rowntree said of the bag hitting the tail rotor. “Now we need to figure out why this happened and how this happened,” Rowntree said.

Hydro One identified the passengers as Jeff Howes, Darcy Jansen and Kyle Shorrock. The pilot killed was James Baragar.

It appeared that two of the passengers were not wearing their seatbelts at the time of the incident.

A third passenger was wearing a seatbelt but was still found outside the aircraft. A piece of the tail rotor was found embedded in the canvas bag that struck it.

Rowntree said Transport Canada rules indicate passengers must wear seatbelts during takeoff, landing and any other time when ordered by a crew member.

Hydro One rules dictate passengers in helicopters must wear seatbelts at all times.

As a result of the investigation, the TSB has reminded all helicopter operators in the country about the importance of securing items outside the aircraft, and the safety risk of not wearing a seatbelt.

Rowntree said the TSB has completely ruled out weather conditions as a factor in the crash.