An improperly secured external bag came loose and struck the tail rotor of the helicopter that crashed last week near Tweed, Ont., killing all four people on board, the Transportation Safety Board says.

The board issued its latest findings Thursday afternoon into the Dec. 14 crash that killed the pilot and three workers, all part of a Hydro One work crew.

The men who died were 39-year-old James Baragar, the pilot, along with powerline technicians Jeff Howes and Darcy Jansen, both 26, and Kyle Shorrock, 27. They were all from either eastern or southern Ontario.

At the time of the crash, the crew was performing routine maintenance work on a hydro line and at a transmission tower on a rural property about 40 kilometres north of Belleville, Ont.

TSB investigator-in-charge Peter Rowntree had previously said the helicopter was flying at a low altitude and preparing to land in a nearby field when it suddenly departed from its flight path.

On Thursday, Rowntree shared the TSB's preliminary findings, revealing that not only had the external bag — which was filled with tools and other supplies — come loose and struck the helicopter's tail rotor, but that two of the three passenger seatbelts were also unfastened.

"We recognize that this is an extremely difficult time for the families and friends of the victims, especially at this time of year," Rowntree said.

"However, when we uncover serious safety deficiencies during the course of our investigations, we do not wait until the final report is published to make them publicly known."

helicopter crash tweed bag tail rotor tsb ottawa dec 21 2017

The Transportation Safety Board says a white canvas bag, at left, came loose from the fuselage of the helicopter, and struck the aircraft's tail rotor, at right. Four Hydro One workers were killed in the crash. (Transportation Safety Board)

Bags normally locked down

Normally, Rowntree said, the bag would have been secured with a double-lock carabiner on a platform extending from the helicopter's fuselage.

However, after the Dec. 14 crash, Rowntree said investigators found a heavily damaged white canvas bag — with a damaged carabiner attached — along with the tip of a tail rotor blade roughly 600 metres from the crash site.

Damage to the tail rotor, he said, would have seriously affected the pilot's ability to counterbalance the power from the main rotor — and therefore steer the aircraft.

"When we lose a piece of that tail rotor, it causes a massive vibration in the helicopter. The pilot's going to feel that vibration in his feet," Rowntree said.

"He's going to know something's wrong."

Shortly after the bag struck the rotor, the three passengers became "separated" from the helicopter while it was airborne, he added.

The helicopter crashed shortly afterwards in a wooded field, he said.

Ont Helicopter Crash 20171214

OPP officers at the Hydro One helicopter crash site in Tweed, Ont., on Dec. 14, 2017. Four Hydro One employees were killed when the helicopter crashed on a rural property in the municipality of Tweed, Ont. (Lars Hagberg/Canadian Press)

Aviation industry warned

The TSB also issued an advisory Thursday to Transport Canada, the Helicopter Association of Canada, and other Canadian aviation agencies warning of the "risks associated with unsecured cargo and unrestrained passengers in helicopters."

The advisory urges helicopter operators to ensure cargo is safely secured and warns that passengers who don't wear seatbelts "risk serious injury or death" if an emergency occurs.

Rowntree said while it was too early to say how widespread issues around properly securing cargo were in the industry, the TSB felt the advisory was needed now.

"It doesn't affect just Hydro One. It effects every single helicopter operator in the country who's doing any kind of aerial work," he said.

"That's news choppers, police, air ambulance — anybody who could be carrying equipment. They need to be sure that it's properly secured at all times. And that's why we want to get this message out now."

Hydro One also has a policy that requires helicopter passengers wear their seatbelts at all times, Rowntree said.

Complex investigation ahead

The helicopter was a 1999 Aerospatiale AS 350 B-2,  registered to Hydro One Networks Inc.

It has has now been sent to the TSB 's engineering laboratory in Ottawa to examine its flight controls and engines in a controlled environment, Rowntree said.

We need to gather all the facts, go through them thoroughly ... that's going to take time. - Peter Rowntree, TSB investigator-in-charge

In the coming days and weeks, Rowntree said investigators will be interviewing witnesses and family members, poring over maintenance records and pilot training, and examining any previous incidents involving that particular helicopter model.

That investigation, he said, will be "complex."

"We need to gather all the facts, go through them thoroughly and come up with a report. That's going to take time," he said. "We'll take whatever time is required to do that thoroughly and make sure that the report is accurate."

CBC News has already learned that Baragar, the pilot, had an excellent record and proper training.

Hydro One chief operating officer Greg Kiraly previously said the company had never had a crash of this magnitude in its history, and that he was "heartbroken" by the deaths of the four men.