AUSTIN (AP)–Schools across the U.S. are warning about a scam to convince parents that their children have been kidnapped — even though they haven’t — and to collect ransom money.

Cases of “virtual kidnapping” have been reported over the past two months in Virginia, California, Texas, Arizona and other states. Authorities say the scam often targets the parents of college students, tricking some into paying thousands of dollars and appears to be on the rise nationally.

In many cases, parents receive a call from a stranger who claims to have kidnapped their child, and can often provide the child’s name or other details. Some parents have reported hearing screams or a muffled cry in the background. Then the caller orders parents to wire money in exchange for their child’s release.

“They really prey on people’s fears, and in this case it’s a very intense fear, thinking that your child’s been kidnapped,” said Jay Gruber, police chief at Georgetown University, where a parent reported the scam on Thursday. In that case, the parent used social media to contact the child, and didn’t pay the ransom.

Usually, the ransom demand is between $600 and $1,900, according to the FBI’s New York field office, which issued a warning about the scam in January 2015. FBI officials said they weren’t available to comment on Friday. Gruber said the scheme emerged in the U.S. more than a year ago but has become more common recently.

Thirkel Freeman was driving with his wife, Coretta, last week when a man called Coretta’s cell phone and said he had kidnapped their daughter, Kiauna, a senior at the University of Maryland. The caller even put a woman on the phone who claimed to be Kiauna and had a similar voice, pleading them to pay the ransom. The man threatened to kill Kiauna if they didn’t.

“He says, ‘If you play games with me, it’s over,’” said Thirkel, of White Plains, Maryland. “At that point, we were at the peak of traumatization.”

Coretta called the police, who arrived and guided the couple through the call. But the Freemans ultimately wired $1,300 to the caller before finding out Kiauna was safe on campus.

Several colleges have issued alerts about the scam, including Georgetown, Arizona State University, George Mason University and the University of Texas at Arlington.

The calls often come from outside area codes, sometimes from Puerto Rico, according to the FBI. If someone calls demanding a ransom, authorities say parents should try to text their child or reach them through social media to confirm their child’s safety. Or they can ask the alleged kidnapper to have their child call back from his or her own phone.

“Once you find out that your child is fine, just disengage with them,” Gruber said. “Or, if your child is with you, tell them to go to hell and hang up on them.”