There may be new hope in the case of the 15-year-old New Hampshire girl who went missing on Oct. 9.

On Friday, the FBI, New Hampshire Attorney General's Office and the Conway Police Department announced that the missing teen, Abigail Hernandez, mailed a letter to her mother, reports NBC affliate WCSH 6.

New Hampshire Assistant Attorney General Jane Young says the letter was dated Oct. 22 and postmarked Oct. 23, but Hernandez's mother did not receive it until Nov. 6.

Young called the letter a "critical lead" in the case, although she did not disclose the contents of the letter or give an explanation for why it took so long to arrive, the Boston Globe reports. She did say, however, that investigators authenticated the letter and that "the tone was one Abby would have used."

Outside of the letter, no one has heard from the missing teen. However, investigators said they are not giving up on the search, while Kieran Ramsey of the FBI assured that the agency will continue to do everything it can to find Hernandez.

Officials also urged the public to be on the lookout for new faces within their community. "Look at your neighbor. Look at the grocery store. Look at your church. Is there someone ... you haven't seen before," said Associate New Hampshire Attorney General Young.

The teen was last seen Wednesday, Oct. 9 after leaving Kennett High School in North Conway. Police say that while she was walking on her normal route home, she sent several text messages to a friend between 2:30 p.m. and 3 p.m. However, she never made it home, reports the AP.

The FBI is offering a $20,000 reward for information leading to a conviction in the missing person case.