(CNN) -- Federal and state authorities won't say specifically why, but they think a letter purported to be from a New Hampshire teenager missing for nearly two months might in fact be the real thing.
Abigail Hernandez was last seen on October 9, according to the FBI, as she headed home from her high school in the town of Conway, and officials fear she might be held against her will.
"We are concerned for her safety," New Hampshire Associate Attorney General Jane Young said Friday at a news conference called to discuss the search for the girl.
"She is not out there alone. She has somebody who is either helping her, whether that be a friend or what we fear is a foe."
At the news conference, authorities revealed that Abigail's mother, Zenya Hernandez, received a letter on November 6 that appeared to be from her missing daughter. That letter was kept secret until Friday, said Young, because "law enforcement had to take every possible step to verify its authenticity.
"And at this juncture, we believe in fact that it was written by Abby and was sent to her mother," Young added.
Neither federal nor state authorities would discuss the contents of the letter beyond Young saying the writing is in "a tone Abby would have used," and that the letter underwent "expert analysis" before its existence was announced publicly.
Officials also said revealing details about the letter could potentially trigger "copycat" letters that would slow down the investigation.
The letter was written on October 22, and postmarked on October 23, according to Young, and it was turned over to authorities for investigation after Zenya Hernandez received it nearly two weeks after it was postmarked.
"When we received this letter I will tell you it was unprecedented. We have not seen anything like that in recent times in other investigations, but most importantly, it gave us hope," FBI Special Agent Kieran Ramsey said at the news conference.
Neither the girl's mother nor the rest of her family has had any contact with Abigail since the letter's arrival, said Young.
Authorities said Abigail had no source of income, and on the day she disappeared she did not have winter clothes with her. Police released photos of a pocketbook and a necklace that were believed to be with the teen when she went missing.
Officials from several law enforcement agencies asked that members of the community be vigilant and pay attention to their surroundings, in the hopes that someone will spot Abigail and alert authorities.
"We are asking you look at your neighbor. Look at the girls next door. Look at your church. Is there somebody that you haven't seen before?" Young said on Friday.
Initially authorities believed Abigail's disappearance may have been an abduction, and continue to believe that she may be being held against her will, but nevertheless they are continuing to classify it as a missing persons case, as all possibilities have not yet been ruled out, according to Ramsey.
Information and tips on Abigail's disappearance are still flowing in from members of the community and law enforcement officials asked that anyone with knowledge of Abigail's whereabouts contact authorities.
"We need to reunite this child with her family," concluded Young.