The FBI and Canadian officials joined the search for the biological mother of adopted twin toddlers who disappeared from North Carolina after a holiday visit and may be in Canada, the FBI said.

The biological mother, Allison Lee Quets, 49, had visitation rights as a part of a custody agreement, authorities said.

The custody order allowed her to take the children from their adoptive home for a brief visit Dec. 22-24, but authorities say she never returned them. An investigation indicated Quets had crossed the Canadian border with the twins on Dec. 23, the FBI said in a written statement Thursday.

Quets now faces federal charges of international parental kidnapping. A warrant was issued Wednesday, according to the statement.

The children, 17-month-old Tyler and Holly Needham, lived in the Raleigh suburb of Apex with their adoptive parents, Denise and Kevin Needham, while Quets had a home in Durham.

According to Quets' sister, Quets never wanted to give up the children but had a rough pregnancy and was worried that she would be too weak to care for them. Through a friend, she met the Needhams, Gail Quets told WRAL-TV.

"They pressured her into giving up custody of her children," Gail Quets said. "She realized less than 12 hours later that it was a horrible decision and she begged to have her children returned to her, and these folks refused."

"I know she loves her children very much and would never endanger them," Gail Quets said.

The Royal Canadian Mounted Police, the Durham Police Department, the National Center for Missing and Exploited Children and Child Find Canada are also involved in the search.