Four Hydro One employees died when their helicopter crashed on a rural property in eastern Ontario Thursday afternoon.

The one pilot and three crew members were part of a crew performing routine maintenance work on a hydro line and at a hydro tower on a property in the municipality of Tweed, Ont., when the aircraft went down, Hydro One spokesperson Ferio Pugliese confirmed.

Hydro One has not released the names of the victims pending the notification of next of kin, but Pugliese said they were based in different regions from across the province.

Kim Clayton, witness to helicopter crash, Dec. 14, 2017

The crash happened on Kim Clayton's property on Upper Flinton Road. 'Nobody should have to lose a loved one at this time, or any time of year, let alone when they go to do a job,' she says. (CBC News)

The crash happened shortly before noon ET on Kim Clayton's property on Upper Flinton Road.

Clayton said workers had been working on a hydro tower all week, with helicopters going back and forth, and sometimes landing on a field on the property or dropping workers off at the tower.

She was inside her home when she heard the crash and felt her house shake.

She looked outside to see other hydro workers running toward the tree line. She couldn't see the helicopter, nor could she see any smoke or fire, but she said she saw what looked like a piece of the helicopter in one of the trees.

It took ambulances about 15 minutes to arrive at the remote rural location, about 40 kilometres north of Belleville.

Tweed helicopter crash Ontario

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

'These guys didn't make it'

"When the ambulances weren't leaving [the crash scene], I kind of pieced it together … these guys didn't make it," she said tearfully.

"Four guys die here today on the property. It's pretty shocking."

"Nobody should have to lose a loved one at this time, or any time of year, let alone when they go to do a job," she said.

An Ornge air ambulance was dispatched at 11:53 a.m. and a second air ambulance was dispatched at 12:15 p.m. Both helicopters were later called off.

Police with the Ontario Provincial Police Central Hastings detachment were also among the first to respond to reports of the crash, according to OPP Const. Lisa Robson. The OPP emergency response team and other units also responded to the scene.

Ont Helicopter Crash 20171214

OPP and Hydro One trucks stand near the crash site. (Lars Hagberg/Canadian Press)

Hydro One has eight helicopters in a fleet it dispatches to perform work along hydro lines in remote locations. It's the first time one of its helicopter crews has been involved in a fatal incident, Pugliese said.

Pugliese added grief counsellors are being made available to employees and their families.

"We're a family here at Hydro One," he said. "This has certainly touched us, it's been a very remorseful day and we're spending a fair bit of time providing the necessary support to our people within the field and throughout the operation and network."

Energy minister Glenn Thibeault offered condolences to "the families of those lost in the crash" and said the province would be monitoring the situation closely and providing updates as they become available.

Ont Helicopter Crash 20171214

Hydro One says it is making grief counsellors available. (Lars Hagberg/Canadian Press)

TSB investigating

The Transportation Safety Board of Canada (TSB) has been notified and three of the agency's investigators have been dispatched to investigate.

The helicopter was a 1999 Aerospatiale AS350 B2, according to Alexandre Fournier, a spokesperson with the TSB.

Investigators will take pictures of the aircraft and gather as much data as possible. They'll also interview potential witnesses as part of the investigation.

"Further along the investigation, they'll consider human factors, they'll interview the operators — the company — examine the wreckage, consider the weather conditions, the maintenance of the aircraft and any other relevant information that can help them to understand exactly how this accident happened," Fournier said.

CBC News has learned weather was not a factor in the crash, meaning investigators will likely be looking at either mechanical failure or pilot error.

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

The helicopter remains at the scene. Investigators told the CBC's Katie Simpson that they will protect the crash site overnight and resume their work in the morning.