A helicopter that crashed in eastern Ontario killing all four crew members was attempting to land when it "departed from controlled flight" and crashed in a wooded area, according to the lead investigator with the Transportation Safety Board.

The one pilot and three crew members were killed while performing routine maintenance work on a hydro line and at a transmission tower on a property in the municipality of Tweed, Ont., when the aircraft went down.

Peter Rowntree, Jean-Pierre Regnier, TSB

TSB Investigator-in-charge Peter Rowntree and investigator Jean-Pierre Regnier say the helicopter was flying at a low altitude as part of its landing when it crashed into a wooded area. (Natalie Nanowski/CBC)

The helicopter was at a low altitude preparing to land at a nearby field when it departed from controlled flight, said Peter Rowntree, the TSB's investigator-in-charge, at a news briefing in Tweed on Friday.

He said weather did not appear to be a factor.

Rowntree said the helicopter was not equipped with, nor was it required to carry, a cockpit voice recorder or a flight data recorder. However, they have recovered a GPS recorder, which they hope will provide clues as to why the crash happened.

The wreckage remains at the scene as investigators gather information, Rowntree said. Later, it will be transported to the TSB Engineering Laboratory in Ottawa to examine the helicopter systems like flight controls and engines in a controlled environment.

The bodies of the four crew members have been removed from the scene and taken to the coroner's office in Ottawa.

Hydro One has not released the names of the victims until next of kin are notified. The workers were based in different regions from across the province.

"We are shaken and heartbroken over the tragic events of the last 24 hours," said Hydro One chief operating officer Greg Kiraly at the news conference.

The workers were part of a crew replacing the shield wire on the hydro lines, Kiraly said. The shield wire is the topmost wire on the lines and protects the other wires from lightning strikes, he said. 

He added that the company does not know what caused the crash but is co-operating with safety investigators.

Hydro One said workers would pause Friday "to take time to grieve and remember four members from our Hydro One family," the company said in a statement via Twitter.

The owner of the property on Upper Flinton Road where the work was being done, Kim Clayton, said she heard a loud crash and saw workers rush to the tree line. She said she didn't see any smoke or fire but saw a piece of what looked like the plane in one of the trees.

Tweed helicopter crash Ontario

The helicopter crash occurred northeast of Tweed, about 190 kilometres west of Ottawa. (CBC)

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

The pilot had an excellent record, proper training and no issues of concern, CBC News has learned.

Hydro One has eight helicopter crews in its fleet that aid in performing maintenance and repairs to hydro lines in remote areas of the province.

Hydro One has never had a crash of this magnitude in its history, Kiraly said.

TSB records show Hydro One did have a previous incident involving a helicopter crash in June 2007 in Moosonee, Ont., when the pilot and a second hydro worker were seriously injured. There were no deaths.

The helicopter became entangled in cables and crashed to the ground. A TSB investigation revealed the cause of the crash was pilot error.

In that 2007 case, the helicopter was flown in a left sideward direction, preventing the pilot from viewing the approaching tower guy wires from his position in the right front seat, the TSB found. The helicopter struck the guy wires supporting the communications tower.

Ont Helicopter Crash 20171214

OPP officers gather at the Hydro One helicopter crash site near Tweed, Ont., on Thursday. (Lars Hagberg/Canadian Press)