(NewsNation) — A California teen who has been missing since Jan. 1, is believed to be alive and possibly held captive, a private investigator says.

Sixteen-year-old Alinka Castaneda left her family’s home in Carson, California, and was seen “entering an unknown vehicle” at 5 a.m. that day, according to the Los Angeles County Sheriff’s Department. A neighbor’s camera recorded her leaving, and her family hasn’t seen her since.

While her brother thought perhaps this was her first attempt at running away, time and distress calls appear to be signs of how much danger she could be in.

Castaneda’s family has received three disturbing calls from her.

“In this phone call, she actually shared with her sister and her mom that she didn’t know where she was then and wouldn’t be allowed to leave. She sounded like she was under duress and stressing out,” Moses Castillo, a retired LAPD detective and a private investigator working on her case, told NewsNation.

The Sheriff’s Department said the girl contacted her family on Jan. 19, saying “she was unable to leave her location.”

The Sheriff’s Department described the teen as an “at-risk missing juvenile” and asked the public for help in finding her.

“There is concern for Ms. Castaneda’s well-being,” the department said.

Castillo said they know the suspected kidnapper is at least one adult male, but there could be more. He also said the suspect’s name is known but not being released yet.

Castillo shared this message to the suspected kidnapper on NewsNation Prime:

“Dude, we know you have her. You’re an adult male; it’s time to just give her up. Bring her to 1200 North State Street in front of the Los Angeles County USC Medical Center; drop her off. You can find me easily on Instagram. Just let me know where you’re going to do that, and I’ll be there to receive her to get her the help she needs.”

If they don’t hear from the man, Castillo said, the next steps would be to go to the police, and they’re working closely with investigators.

“There’s no teenager that could live on her own without any support from anybody,” he said.

One main concern is that Castaneda is a victim of human trafficking, but Castillo said, “There’s no confirmed information about that.”

Castillo said Castaneda’s family just wants her home.

“They’re going to support you and your recovery,” he said, addressing the teen. “And there’s not going to be any hurt feelings, any harm feelings. But just come home. We’re here for you.”

After a vote, the local city council confirmed a $100,000 reward for information leading to Castaneda’s return.

The teen is 5 feet, 5 inches tall and 150 pounds, and recently had short brown hair and brown eyes. Her ears and nose are pierced.

Anyone with information about Castaneda should call the Sheriff’s Department at 310-847-8362.