The wreckage of a helicopter is shown at the Transportation Safety Board of Canada's lab in Ottawa in this recent handout photo. The Transportation Safety Board says evidence suggests a tool bag being carried outside a Hydro One helicopter struck the rear rotor of the aircraft just prior to a crash in eastern Ontario that killed all four people aboard. THE CANADIAN PRESS/HO - Transportation Safety Board *MANDATORY CREDIT*

An improperly secured tool bag hit the rear rotor of a helicopter before the aircraft crashed in eastern Ontario, killing four people on board, the Transportation Safety Board said Thursday, as it issued a warning on the risks of unsecured cargo.

The board said its preliminary investigation into the Dec. 14 crash near Tweed, Ont., suggested the tool bag being carried on a platform outside the Hydro One helicopter was the main cause of the deadly incident.

Shortly before the crash, the helicopter's pilot picked up three linemen at the base of a high-power transmission tower and was transporting them to a nearby staging area, the TSB said.

A few bags used for tools and supplies were being carried on the platform extending out of the right side of the helicopter. The bags are normally secured with double-lock carabiners, the TSB said.

"While nearing the staging area, one of the bags ... blew off the platform and along with its attached carabiner struck and damaged the tail rotor," Peter Rowntree, a senior TSB investigator told reporters on Thursday.

"A heavily-damaged carabiner, a damaged bag along with the tip of a tail rotor blade were found approximately 600 metres away from the crash site."

Investigators also found that two of the three seatbelts in the helicopter's passenger area were unfastened, Rowntree said.

"All three passengers became separated from the helicopter while it was still airborne," he said.

Rowntree said the TSB investigation into the crash continues and will examine helicopter maintenance records, pilot training, operational policies and previous occurrences involving the helicopter model involved.

Weather is not being considered a factor in the crash, he said.

The TSB issued a safety advisory Thursday in light of the crash, saying cargo must be adequately secured at all times and warning that passengers who don't wear seat belts risk serious injury or death in an emergency.

The men killed in the crash were identified by Hydro One as 39-year-old James Baragar, 27-year-old Kyle Shorrock, and Jeff Howes and Darcy Jansen, both 26.

Hydro One has said Baragar, the pilot, had been with the company since 2009.

Greg Kiraly, Hydro One's chief operating officer, said Thursday that the utility is treating the matter with "the utmost seriousness."

"We are pleased that the TSB has been able to release this preliminary information and we will be incorporating today's safety bulletin into our own review of our operating practices and procedures," Kiraly said.

The helicopter did not have a cockpit voice recorder or a flight data recorder, but the TSB has said it recovered a GPS, which will help in the investigation.

News from © The Canadian Press, 2017