VANCOUVER -- Two Alberta girls are safe and their mother has been arrested after an Amber Alert issued Wednesday morning garnered public tips that helped police locate the trio in British Columbia's Fraser Canyon.

By evening, RCMP said girls, ages 6 and 12, were located in Boston Bar, B.C., about 215 northeast of Vancouver.

Their mother, 48, who does not have legal guardianship, faces two counts of abduction.

Acting on information from the public, police said they located the three and took them into custody without trouble.

"The two girls are OK and investigators are working with the Ministry of Children and Family Development with respect to their care," B.C. RCMP Sgt. Rob Vermeulen said in a news release.

The alert was cancelled just hours after police in Alberta warned that their investigation led them to believe the girls could be in grave danger.

Mounties first issued a warning Monday after the girls were discovered missing just before 11 a.m. MST. An arrest warrant was then issued on Tuesday in connection with what was believed to be a parental abduction.

On Wednesday morning, police heightened the search to an Amber Alert after a "confirmed sighting" they said occurred Monday at 1 p.m. PST in the border town of Golden, B.C.

RCMP Insp. Gibson Glavin, a senior officer in southern Alberta, said later in the day that the scenario hadn't met all criteria required to activate the Amber Alert for two days.

"We did not have one of the four criteria, which was belief that the children were at risk of serious bodily harm or death," he said at a news conference not long before the group was located.

"What changed was (Wednesday) morning, information came to the investigator's attention that caused him to believe the children could be at risk of serious bodily harm or death."

At the time he did not elaborate.

Police began investigating after the girls disappeared from their elementary school in Brandt, near Vulcan, Alta., south of Calgary.

It was believed they sneaked out of school to join their mother, who is the non-custodial parent.

Police issued a plea for the woman to come forward, when she was believed to be travelling across B.C., possibly towards the Lower Mainland or Vancouver Island.

RCMP Insp. Tony Hamori, officer in charge of major crimes in southern Alberta, said police in Alberta had been communicating extensively with all police forces in western Canada since the investigation began.

The RCMP detachment in Ladysmith, on the southeast coast of Vancouver Island, was first advised the family could be headed west on Tuesday based on information posted on Facebook.