Denver Zoo Director of Communications Tiffany Barnhart says Alzono Ashley, 29, appeared to be passed out near the elephant exhibit shortly before 5 p.m. on Monday.
When a security guard was called, Ashley had placed his head under the drinking fountain and was allegedly making bizarre statements.
"He claimed he was a lion," Barnhart said.
When the security guard approached Ashley's girlfriend, Barnhart says the man began shouting and threatening the unarmed security guard and knocking over trash cans. Then he allegedly chased the security guard, tackled him, and began punching him.
"The guard was attacked and beaten," Barnhart said.
Zoo officials then called Denver Police, who later used a stun gun on Ashley and he collapsed. Police officers attempted CPR and Ashley was taken to the hospital. He was pronounced dead around 6 p.m.
"This is a very difficult week for the Denver Zoo," Denver Zoo President/CEO Craig Piper said.
Piper also expressed sympathy for Ashley's family and thanks for "an outpouring of kindness and support" for the zoo.
Barnhart called Monday's death "an isolated, unprecedented" incident in the history of the Denver Zoo.
The Denver Office of the Medical Examiner performed an autopsy on Wednesday, but said the cause of death for Ashley is still pending and needs further investigation.
Police say drugs and drug paraphernalia were found on Ashley.
His family disputes the claim that he was fighting against officers during the confrontation.
Police are still looking for witnesses to the incident and are asking them to call 720-913-6050.
(KUSA-TV © 2011 Multimedia Holdings Corporation)