In this Monday, July 18, 2016, photograph, Denver Police Department officers head out to their patrol car after role call for the swing shift in District 6 in downtown Denver. After more than two decades of making officers patrol on their own, the city of Denver has revived two-officer patrols following the killings of officers in Dallas and Baton Rouge, La. (AP Photo/David Zalubowski)

Denver police are investigating the theft of a box containing human heads from a parked freight company truck, officials said Saturday.

The box was being transported for medical research purposes, police said in an email.

Someone broke into the truck while it was parked between Wednesday afternoon and Thursday morning, Denver television station KDVR reported.

The blue-and-white box was marked with a label that said “Exempt Human Specimen,” police said. A dolly was also stolen from the truck.

No arrests had been made as of Saturday and investigators asked anyone who finds the box to call police.

The Associated Press