Human remains have been found in two of the three bears euthanised after they were suspected of attacking and killing a woman, according to wildlife officials.

The 39-year-old woman's body was found north of Durango in southwestern Colorado on Friday.

Authorities suspected she had been the victim of a rare bear attack due to the bear scat and hair found at the scene.

A dog team found a 10-year-old female black bear and two yearling cubs nearby. They were killed after it was suspected they attacked the woman and could attack someone else.

The human remains were found inside the stomachs of the sow and one of the cubs during a necropsy on Saturday, Colorado Parks and Wildlife (CPW) said.

An initial examination by a pathologist showed the bears all appeared to be in good health with no sign of disease and adequate fat stores following the winter hibernation season, according to the agency.

It added that a microscopic examination of their body tissue and rabies testing to find other possible reasons that could have prompted the attack will take up to two weeks.

In the announcement of the necropsy findings, Cory Chick, CPW's regional manager, said: "Our thoughts and prayers go out to the boyfriend, family and friends of the woman we lost in this tragic event."

"We cannot determine with exact certainty how or why this attack took place, but it is important for the public not to cast blame on this woman for the unfortunate and tragic event."

The woman's boyfriend returned home at around 8.30pm on Friday and their dogs were outside, but she was missing after taking them out for a walk.

He began to search and found her body an hour later off US Highway 550 near Trimble, a community north of Durango, and called the emergency services.

A post-mortem will take place on Tuesday.

The last fatal bear attack in Colorado was recorded in 2009 near Ouray - when a 74-year-old woman was killed by a 394lb (179kg) male black bear.

An investigation determined she had been illegally feeding bears through the fence in her yard, CPW said.