A Canadian-owned C-130 Hercules air tanker crashed while fighting bushfires in Australia’s remote southern alpine ranges on Thursday, killing all three of its American flight crew.

The NSW State Premier Gladys Berejiklian confirmed the crash deaths in the state’s Snowy Monaro region, adding few details are currently available as to the circumstances behind the tragedy.

Flight tracking made available later in the day showed the course of the downed aircraft:

RAAF A47-005 P-8A Poseidon as “BLACKCAT 50” currently searching for a missing C-130 Large Air Tanker in the Snowy Mountains region of NSW, Australia. NSW RFS confirmed that a C-130 is currently missing and a SAR operation is taking place. #nswrfs #c130 #coulson #nswfire #aviation pic.twitter.com/n7uhETZhEc — scanSydney (@scanSydney) January 23, 2020

Coulson Aviation in the U.S. state of Oregon confirmed one of its Lockheed large air tankers left Richmond air force airbase with retardant for a firebombing mission and radar contact was lost over rugged bushland near the national capital Canberra.

It said the accident was “extensive” but had few other details.

Coulson has grounded its large air tanker fleet following the accident, pending a review to rule out any systemic issue, such as a fuel problem.

“The only thing I have from the field reports are that the plane came down, it’s crashed and there was a large fireball associated with that crash,” said Rural Fire Service Commissioner Shane Fitzsimmons.

Foreign Minister Marise Payne said she had conveyed Australia’s condolences to U.S. Ambassador Arthur Culvahouse Jr.

“Our hearts go out to their loved ones. They were helping Australia, far from their own homes, an embodiment of the deep friendship between our two countries,” she said in a statement.

“Thank you to these three, and to all the brave firefighters from Australia and around the world. Your service and contribution are extraordinary. We are ever grateful,” she added.

Culvahouse in turn Tweeted his statement:

Statement by Ambassador Culvahouse: pic.twitter.com/w23ioPSZxa — US Embassy Canberra (@USAembassyinOZ) January 23, 2020

While details remain scarce, one theory is the plane may have been “aerodynamically overloaded”, said Geoffrey Dell, an expert in accident forensics and investigation, told Reuters.

This meant it may have turned or changed altitude too quickly for its load, forcing it to lose altitude sharply.

“It’s designed to operate to certain G (gravity)-limits, and if you go over those, different parts of the structure can be overstressed,” added Dell, a professor at Central Queensland University.

The missing plane was a re-purposed C-130 Hercules, built in 1981 in America by Lockheed Martin.

AP contributed to this story