Where Was Missing Teen Abigail Hernandez for the Past 9 Months?

— -- Questions surround the return of a teenage girl who disappeared on her walk home from school nine months ago.

Abigail Hernandez vanished when she left her school in Conway, New Hampshire, on Oct. 9 and, aside from one letter to her mother this winter, there had been no indication by officials that there was any trace of her until she was reunited with her parents on Sunday.

Investigators were remaining tight-lipped about the 15-year-old's return, refusing to even say how the girl was reunited with her mother, Zenya Hernandez.

9 Months After Vanishing, New Hampshire Girl Is Home

The criminal investigation into the teen's disappearance has not stopped, authorities said.

"We have said from the beginning and we continue to believe that this was a 14-year-old girl who went missing in October," Associate Attorney General Jane Young told ABC News. "She was gone for nine months. She didn't have the means to orchestrate and facilitate this on her own."

Abigail's whereabouts during her disappearance remain a mystery, with the only two locations publicly connected to the case being her school, Kennett High School in North Conway, New Hampshire, and her mother's home in Conway.

According to the website established to bring attention to her case, Abigail was last seen walking on a path near a power line near her school at 2:30 p.m. on Oct. 9. Her final text to a friend was sent at 2:57 p.m. that afternoon and her cell network noted last contact with the phone at 3:07 p.m.

She did not have money, IDs or any additional sets of clothes with her when she went missing, according to the website.

The biggest break in the case appeared to come in November when, according to officials, her mother received a letter from Abby postmarked in late October.

"Our worst fear right now is that while she could have left willingly, someone may now be coercing her, someone may now be manipulating her," FBI Special Agent In Charge Kieran Ramsey said in a December press conference detailing the letter. "When we received this letter, I'll tell you it was unprecedented. ... It gave us hope."

That hope was rewarded Sunday when Abigail was reunited with her sister and mother, who said, "Today, we are the happiest people on earth."