Photo: The Canadian Press

Police are crediting an off-duty officer from Calgary with spotting an Edmonton boy at the centre of an Amber Alert that spanned two Alberta cities.

Edmonton police say the Calgary officer saw the boy and his mother thanks to the Amber Alert, and that RCMP from Okotoks took the woman into custody.

"It was without incident," Edmonton police Staff Sgt. Ashley Emerson said of the arrest.

Police say the boy's mother has been charged with one count of abduction, and the eight-year-old boy has been returned to his father.

When they issued the alert Friday night, police said they believed the boy was abducted from school by his mother that afternoon, and that he was believed to be in danger.

Emerson said police were alerted by a parent at around 3:50 p.m. that a boy was walked out of the building by a woman, without his coat or his outdoor footwear. The school had notified the parent, Emerson said, who then called police.

After leaving Edmonton, the pair were believed to be travelling in the Calgary area.

Emerson said investigators didn't know at first who the woman was. They then learned there was a parenting order in place and suspected the woman might be the non-custodial parent.

He explained it was several hours before an Amber Alert was issued because police needed to be satisfied that the child's safety was at risk.

"It was only at that point that we utilized that system. We don't use it sort of as an investigative tool to help us resolve the matter quickly. We use it when we're satisfied that it's met the criteria and that the child is in fact in danger," Emerson said.

Emerson said he didn't know details of how the Calgary officer knew an alert had been issued for the boy. He said the officer spotted the vehicle that was described in the alert heading east out of the town of Black Diamond on Highway 7 and followed it.

RCMP from Okotoks pulled the vehicle over on Highway 7 about halfway between Black Diamond and Okotoks, he said.