The FBI said Thursday that 17-month-old twins allegedly taken by their birth mother last week were spotted crossing the Canadian border.

According to a news release issued early Thursday evening, FBI investigators have determined that Allison Quets crossed the border last Saturday with the toddlers, Tyler and Holly Needham. All three are believed to still be in Canada, investigators said.

The twins were taken from their adoptive parents in Apex last Friday for a scheduled monthly visitation with Quets. Police said Tuesday that Quets, 49, never returned Sunday evening.

On Wednesday, a federal warrant was issued charging Quets with violating an international parental kidnapping law. Additionally, federal investigators filed a provisional arrest warrant application with Canadian authorities to assist in locating Quets and returning the two children to the United States.

FBI officials said they are working jointly with the Royal Canadian Mounted Police, the FBI legal attache's office in the U.S. Embassy in Ottawa, and the Durham Police Department. They also have enlisted the assistance of the National Center for Missing/Exploited Children, along with Child Find Canada.

Quets had been embroiled in a custody dispute with Kevin and Denise Needham , who live in Apex, in Jacksonville, Fla., where Quets lived.

The FBI said that matters concerning custody and parental rights would be determined by the appropriate authorities in the state of Florida.