Authorities revealed on Tuesday that the 15-year-old girl from the St. Louis area had been missing since October.

The unidentified 16-year-old girl from Columbia was miraculously rescued from inside Maximilian Bondrescu’s house in Fort Collins, just outside Denver, in raid as part of a kidnapping probe.

Investigators had received a tip that the teen — who suddenly went silent on social media from Dec. 6 — had been spotted at the home, according to police.

The 44-year-old registered sex offender initially denied the teen was there but a search found her stashed in the closet, cops allege.

Bondrescu is accused of kidnapping the girl after renting a vehicle to pick her up in Missouri.

The teen alleged Bondrescu then held against her will once at his home.

She told cops that Bondrescu would allegedly only sometimes let her out of the house to work for his snow removal company — making her wear a mask to conceal her identity and age.

She was finally rescued in the raid on April 18. It wasn’t immediately clear how Bondrescu initially met his alleged victim.

“The importance of the work done by the Fort Collins Police Cyber Crime Unit, and similar units across the nation play a vital role in helping to keep our children safe,” Fort Collins Police Assistant Chief Kristy Volesky said.

“Had it not been for the diligent work of these detectives, the juvenile could have continued to be victimized. Our investigators will now work closely with the District Attorney’s Office to ensure justice for this child.

Bondrescu was arrested on suspicion of second-degree kidnapping, second-degree assault, sexual assault on a child, false imprisonment, failure to register as a sex offender, child abuse, harboring a minor and obstructing a peace officer.