The FBI and New Hampshire attorney general's office said 15-year-old Abigail Hernandez wrote her mother on Oct. 22. The letter was postmarked October 23, two weeks after she was last seen on Oct. 9, but the family didn't get the letter until November 6.
Senior Assistant Attorney General Jane Young said the delay may have been caused by the letter going to a post office box and not the Hernandez house.
Police said they didn't disclose the letter until they could verify it was authentic. They are not releasing the letter or any of the contents and won't say where it was mailed from.
Law enforcement officials said the letter, which they called unprecedented in similar investigations, has given them hope, though they still have "grave concerns" for her safety.
Abigail was last seen after leaving Kennett High School in North Conway. Police have said she walked her normal route toward home and sent several text messages to a friend between 2:30 and 3 p.m. At first, police said she made it home, but they later said she hadn't. Police also at first said she made a call about 6:30 p.m. that day but later said that, too, was wrong.
The last signal her phone sent was at 3:07 p.m. from the west side of Cranmore Mountain Resort, 2.5 miles from her home.
"Though she could have left willingly, someone now could be coercing her," said Kieran Ramsey, the FBI special agent in charge who is leading the investigation. "Someone now may be manipulating her."
Asked if Abigail had run away before, Young said: "This is not a characteristic that we have seen before from her."
Police asked people to be on the lookout for the dark-haired girl. They said neighbors should ask themselves whether a young woman had recently moved into the area.
Young said because Abigail has no financial means, police believe she's getting help "either from a friend or, as we fear, a foe."
"We implore Abby that if she hears this, if she's able to see this, contact us," she said. "We will do everything we can to reunite her with her mother. If it's a good citizen, a good Samaritan, who has her who thinks you're helping her, I can assure you, you're not."
Since she vanished, police have consistently said they have no evidence to suggest anything suspicious and are treating this as a missing-person's case.
A $20,000 reward and activation of the FBI's tip line, 1-800-CALL-FBI, have yielded a few leads but no solid information. Her family has made pleas for her safe return, and the community has rallied around them, holding vigils, helping search, printing posters and sponsoring billboards seeking information.
