(CNN) -- The two Amish girls kidnapped last week in upstate New York were sexually abused while in captivity, St. Lawrence County District Attorney Mary Rain said.
Police on Friday arrested a couple and charged them with kidnapping in the abduction of the girls last week, authorities said.
Stephen M. Howells II, 39, and his girlfriend Nicole F. Vaisey, 25, are being held without bond, the St. Lawrence County Sheriff's Office said. Federal charges are also a possibility.
Rain said that evidence found in the couple's house, in addition to interviews with the couple and the victims, shows that the kidnapping was premeditated and that the couple had planned for more.
"There is no doubt in my mind that if they were successful, they were going to continue with future acts," Rain told CNN.
The District Attorney's Office is also looking into whether the couple had sexually abused others, Rain said.
On Wednesday of last week, an Amber Alert was issued after a 6-year-old and her 12-year-old sister disappeared from a vegetable stand near their family farm in Oswegatchie, about 50 miles (80 kilometers) from the Canadian border.
A witness told police the girls were tending to a customer who drove up in a white four-door sedan and disappeared when the vehicle pulled away, according to the Amber Alert.
The Amber Alert was canceled Thursday night after the girls were apparently abandoned along a road in Richville, about 40 miles away.
"The children walked up to a home, knocked on the door of a stranger," Rain told CNN affiliate WWNY. "The stranger brought them home to their house, and police were waiting."
She said the girls were "a little wet and cold," but otherwise healthy.
The investigation moved quickly Friday, as "good police work" and tips from the girls led authorities to the suspects, St. Lawrence County Sheriff Kevin Wells said.
"We have two strong young girls," he said. "The reason we're able to make an arrest tonight is because of just how strong they are and things they're able to remember."
A red car and a white one were towed from the scene by authorities, affiliate WSYR reported. The house where Howells and Vaisey live is being processed for evidence, Wells said.
A preliminary hearing for Howells and Vaisey that was scheduled for Thursday was canceled Monday. If convicted, the couple faces a minimum of 25 years in prison on the kidnapping charge.
CNN's Ed Payne and Joe Sutton contributed to this report.