Investigators hope to track down the prank caller who threatened to harm students at Alpharetta Elementary School, police said Tuesday.

At 9:39 a.m., an unknown caller contacted the school and stated he had a gun on the roof and planned to shoot children, according to George Gordon, Alpharetta police spokesman said.

The call prompted a search and lockdown at the school, where about 50 students were attending a summer program Tuesday.

“At this time all students are safe, heavily protected and a sweep of the school and campus continues,” Gordon said in an emailed statement. “No actual threat has been found.”

The lockdown was lifted shortly after noon at the school, located on Mayfield Drive.

“In today’s age it’s absolutely unbelievable that somebody will still do this, especially making a threat against kids,” Gordon told Channel 2 Action News. “It’s incomprehensible to me, to the average person. Hopefully, if we get this person into custody, they’re going to understand that before a judge.”

Gordon said police have an audio recording of the call and are tracking it to find the source.