Authorities went to the building on Clovis Avenue near Ashlan around 1 a.m. Monday. They said several people were inside, when one of them fell about 30 feet from a walkway.
It's unclear what they were doing, but family members say the group was ghost hunting.
"It's a spooky old building but I'm not sure what they were doing. Up to no good, goofing around that's for sure," said Curt Fleming, Clovis Police Department.
The victim, whose name has not been released, was taken to the hospital and is expected to survive. It is unclear if the people will face any trespassing charges.