The desperately worried sister of Olivia Ambrose used the Find My iPhone app to determine the location of her abducted twin, police said.
Fanny Ambrose employed the tracking service to search for Olivia's cellphone, which was pinpointed at the intersection of Corey Road and Walford Way in Charlestown, Boston police said.
The missing 23-year-old woman was found Tuesday in the apartment of her alleged captor, 38-year-old Victor Pena, authorities said. Olivia Ambrose had vanished late Saturday after leaving a Boston bar where she had met her sister and a group of friends.
Surveillance video showed the woman being held and guided toward a transit station, authorities said.
"It's obvious from the video surveillance that she did not go along willingly," Boston Police Commissioner William Gross told reporters at a Tuesday night press conference after Olivia had been rescued.
Officers banged on Pena's apartment door for 20 minutes and were dismantling the locks when he finally opened it, according to an incident report filed by Boston police. Olivia was standing in a hallway, "crying with a horrified look on her face," the report said. Pena "refused to comply and resisted violently," according to the document.
Pena's apartment building is at the intersection where Olivia's cellphone was picked up by the tracking app, officers said.
On Thursday, Fanny Ambrose posted a thank you to all those who helped search for her twin. She also asked for people to take down posts about her sister's disappearance so she wouldn't be bombarded with "reminders" on social media.
On Wednesday, in his first court appearance, Pena cried copiously, sucked his thumb and appeared to pray, The Boston Globe reported.
A judge ordered Pena held for a 20-day evaluation to determine if he is mentally competent to stand trial on kidnapping charges.
Jose Pena told the paper his brother, Victor, is "a little bit mentally challenged."
In a later interview with the Globe, Jose Pena said his brother's mental capacity was severely damaged as a child when a medical problem deprived his brain of oxygen. "Him kidnapping her, I'm 100 percent sure he didn't," Jose said. His brother said the woman had "willingly" come with him, Jose told the paper.