A former teacher charged with multiple counts of sex abuse after one of her accusers went public in a YouTube video confessed to detectives that she molested the former student, court records show.

Riverside police Det. Roberta Hopewell, in seeking an arrest warrant for Andrea Michelle Cardosa, 40, stated that the former Alhambra High School assistant principal "admitted to having a sexual relationship" with the teenager when she taught in Riverside County.

The detective said Cardosa initially stated the girl was 15 or 16 but later acknowledged she may have been as young as 14, according to a copy of the arrest warrant obtained by The Times.

The detectives interviewed Cardosa after the victim posted on YouTube a video of a phone call in which Cardosa "admits" to the sexual relationship, Hopewell said.

Cardosa told detectives the incidents occurred in Riverside and Perris and continued when the victim moved to Victorville and Hesperia in San Bernardino County, Hopewell said.

Cardosa was charged Monday with 16 felony counts of aggravated sexual assault and lewd acts on a child under the age of 14, the Riverside County district attorney's office said. The charges involve two victims.

Cardosa was arrested on a $5-million warrant Monday night in Perris, officials said, and she was expected to be arraigned Wednesday.

Jamie Carillo, 28, who made the YouTube video, said Cardosa began sexually abusing her in the late 1990s, when she was an eighth-grader at a Riverside middle school.

An attorney for the other alleged victim, identified only as Brianna, said his client was abused in 2009 and 2010, when she was a middle-school student in Perris.

Carillo secretly recorded her phone conversation with Cardosa and posted the video Jan. 17 on YouTube. The video was viewed more than 1 million times in a week but has since been taken down.

In the nine-minute recording, Carillo is shown dialing a number using speaker-phone mode. A woman answers, telling her she has reached Alhambra High School. Carillo asks for "Andrea Cardosa" and is redirected twice. Then a woman answers and agrees that she is Cardosa, an assistant principal at the high school.

"You brainwashed me and you manipulated me.... What you did was wrong," Carillo says in the video.

"It wasn't anything I intended," the woman says quietly. "I don't even know what happened."

"You ruined my life. You ruined my childhood. Do you realize that?" Carillo says, her voice raised. "You sicken me. You sicken me. And every day when I think about what you did, you sicken me."

"I regret it every day," the woman says. "Every day."

Alhambra Unified School District Supt. Laura Tellez-Gagliano said school staff received an email with a link to the video the day it was posted and immediately reported it to police. Cardosa resigned a few hours later.

In court documents, detectives said Carillo was 12 when she met Cardosa at Chemawa Middle School in Riverside, where Cardosa became her mentor and began the sexual abuse in the girls locker room.

According to court documents, the abuse continued until Carillo was 18 and occurred "too many times to count."

Within days of the start of the investigation, the second woman, who is now 18, came forward, court records show.

That girl met Cardosa at Tomas Rivera Middle School in Perris and was 14 when Cardosa took her for a ride to a secluded area in Mead Valley south of Riverside and sexually abused her in 2010, Hopewell said.

richard.winton@latimes.com