The strange theft took place between 2:30 p.m. Wednesday and 9:30 a.m. Thursday while the freight company truck was parked in Denver’s Central Park neighborhood, police said. The heads were being transported for medical research purposes, according to authorities.
"Pretty shocking," Isaac Fields, a local resident who routinely walks his dogs in a nearby park, told FOX 31 Denver. "I guess I don’t see too many strange things happening around here usually."
The blue and white box had a label that said "Exempt Human Specimen," police said, according to The Denver Post.
A dolly was also stolen and police did not say how many heads were in the box.
The cardboard box, which was about 20 x 15 x 18 inches, also had "Science Care" written on the sides, sources told FOX 31.
Science Care is a program for donating bodies for both scientific research and education, according to the station.
The investigation is still ongoing, police said, and no arrests have been made as of Sunday.
Anyone who finds the box of human heads is asked to call the Denver Police Department, authorities said.