Two missing special needs teenage twins were found near the University of Southern California, Los Angeles County Sheriff's Department Lt. Richard Erickson told FOX News on Friday.

The 16-year-old girls are safe and back home with their adoptive parents.

"We are so relieved," Kay Corrodi, the girls' mother, told FOXNews.com. "The sheriff called last night saying he had a lead and then called a few hours later saying he got them. It was terrific."

She described the girls as being "OK" and "very quiet" since returning home, but quickly added that was pretty much normal for them.

A 23-year-old suspect is in custody on related charges, according to Erickson. One of the girls returned with a tattoo of the suspect's name on her chest.

The twins, described as mentally slow by their parents, had been missing since Sunday.

They were last seen outside their Malibu house getting into a car occupied by two women.

Jack Corrodi said earlier this week that his daughters have IQs of only 70 and attend special education classes.

They had recently spent "many hours" on the Internet and received multiple calls from unknown people.

He said that the twin sisters are "very slow mentally" but seem much older than 16.

Erickson said this week that police were classifying the teens as runaways, since they had left home in the past.