Of three reported missing "one of those persons has been accounted for alive and well," Boulder County Sheriff Joe Pelle told journalists, adding that a search was ongoing for the remaining two.

Hot debris that has been covered by snow has however hampered recovery efforts, he said.

The so-called Marshall Fire, which was sparked in a tinder-dry landscape, was fanned by winds gusting at more than 100 miles (160 kilometers) an hour on Thursday.

As the blaze raced through the towns of Superior and Louisville, just outside the state's biggest city Denver, tens of thousands of people were forced to flee with little notice.

Shocking aerial footage showed whole streets as little more than piles of smoking ash, destruction that appeared almost total but somehow left a few homes oddly untouched.

The cause of the blaze is still under investigation, with a search warrant in one location announced Saturday.

"There's no news yet on the ongoing investigation into the source of the fire," Colorado Governor Jared Polis told CNN on Sunday. "It's being done professionally in conjunction with the FBI, with forensic expertise around it."

"If there was, of course, anybody responsible for the start of this fire, we expect that they'll be held fully accountable under the law," he said.

A total of 991 structures, the vast majority of which were homes, were destroyed in the inferno, Pelle said Sunday. Some 127 were damaged.

"It's a very difficult search because of the debris, the heat, the snow," Pelle said.

In order to locate the missing, "they're trying to use dogs to help them pinpoint exactly where to work, but it's even difficult to get the dogs into the scene," he said.

FEMA Administrator Deanne Criswell told CNN Sunday that "being able to see it firsthand today on the ground and see the absolute devastation that these communities have experienced is shocking."

Within the burn zone, roughly 1,000 customers remained without electric power Sunday, Alice Jackson with Xcel Energy said, while more than 11,000 remained without gas.

Although fires are a natural part of the climate cycle and help to clear dead brush, their scale and intensity are increasing.

Scientists say a warming climate, chiefly caused by human activities such as the unchecked burning of fossil fuels, is altering weather patterns.