The cause of a fatal air tanker crash that killed three Americans on Thursday is still unclear and will be investigated. The C-130 Hercules firefighting aircraft crashed in the Snowy Monaro region in the afternoon.
It was fighting fires in the area when the NSW Rural Fire Service lost contact with the aircraft at 1.30pm.
All three crew members on board were residents of the United States. NSW Police said three men, aged 42, 43 and 45, were believed to be on board at the time of the crash, but they had not been formally identified.
The aircraft, registration N134CG, crashed at Peak View north-east of Cooma.
RFS commissioner Shane Fitzsimmons said there was no indication what caused the crash.
"It's impacted heavily with the ground and initial reports are that there was a large fireball associated with the impact of the plane as it hit the ground," he said.
"We've got a number of firefighters and a number of crew that are in the area and working to contain and work around the fire.
"It did take some time with the use of ground crews and a number of aerial surveillance platforms to try and locate the wreckage."
It was reported that a nearby plane described the incident as "It's just a ball of flames ... over."
Helicopters flew over the region and the military was deployed to help locate the aircraft after the crash.
Officers from Monaro Police District worked with Fire and Rescue NSW, NSW Rural Fire Service and Australian Defence Force personnel at the crash site.
The Australian Transport Safety Bureau said it would investigate the crash and release a preliminary report in a month. NSW Police detectives will assist investigators.
NSW Premier Gladys Berejiklian said the aircraft was operated by Coulson Aviation, an aerial firefighting company contracted by the RFS.
"It demonstrates the dangerous work currently being undertaken and it also demonstrates the conditions that our firefighters are working under," she said.
"There are in excess of 70 aircraft that have been used today alone and today is a stark and horrible reminder of the dangerous conditions that our volunteers, our emergency services personnel across a number of agencies undertake on a daily basis."
Coulson Aviation said in a statement the aircraft had left Richmond, NSW with a load of retardant on a firefighting mission.
The company is making contact with the families of the crew members and will send a team to the crash site.
"Our thoughts and prayers are with the families of the three crew members on board," it said.
Mr Fitzsimmons paid tribute to the crew members as remarkable and well-respected firefighters who had dedicated many decades of their lives to fire management. They had routinely crewed C-130 Hercules aircraft, he said.
Prime Minister Scott Morrison described the crash as a "terrible tragedy".
"My deepest condolences to the loved ones, friends and colleagues of those who have lost their lives," he tweeted.
Opposition Leader Anthony Albanese said the incident was "another tragic reminder of the extraordinarily dangerous conditions our emergency workers are facing".
Coulson Aviation has grounded its large air tankers as a precaution for the welfare of their other crews operating aircraft in NSW and Victoria. The RFS will investigate for systemic supply issues including fuel that could cause other crashes.
The RFS bought a 737 aircraft from Coulson Aviation in May and contracted the company to operate the C-130 that crashed. Coulson Aviation has another C-130 operating in Victoria.
In a statement the US Ambassador Arthur B Culvahouse JR said he was deeply saddened by the tragic news.
"The brave Americans who died near Snowy Monaro died helping Australia in its time of need," he said.
"The families and friends of those who we have lost are in our thoughts and prayers."
The US Embassy said it was in contact with the families of the victims and was providing consular assistance. The embassy's flag is flying at half-mast on Friday.
Locals in Numeralla, east of Cooma, said they were devastated to learn the Americans had come from the other side of the world and lost their lives protecting their area.
At the local Rural Fire Service shed little was known about the incident, which happened about 24 kilometres north of the tiny village.
Volunteer firefighter Belinda Vlahos said local crews had been working all day to put out multiple fires in the area and three homes had been saved on Countegany Road on the edge of town.
"We didn't even know they were out there," Ms Vlahos said of the air tanker that went down in the remote country.
"We thought you wouldn't be out there on a day like today because of the wind."
No one who spoke to TheCanberra Times saw the plane go down, but Ms Vlahos said some locals believed they had heard it.
Local authorities said it was unknown when police would get access to the crash site, as it was still a fireground and the fire was expected to continue burning in coming days.