(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. 
By whom?
"And at this juncture, we believe in fact that it was written by Abby and was sent to her mother," Young added.