Around 5 p.m., Denver Police officers were called to the zoo for a domestic dispute, but late on Monday evening police said the exact nature of the incident was still being investigated.
Police say zoo security guards tried to break up the fight between the man and woman near the elephant exhibit, and tried to take control of the man.
Shortly after that, security guards got help from Denver Police officers.
At some point police say the suspect stopped breathing.
Denver Police spokesman Sonny Jackson says officers gave the man CPR and he was taken to the hospital where he was pronounced dead around 6 p.m.
"It was kind of scary actually," zoo visitor Jhoselyne Canales said.
She and her friends had no idea the main attraction during their visit at the zoo Monday afternoon wouldn't feature the animals.
"We were coming up from seeing the lions and all of a sudden we see cops and security guards all over the place," Canales said.
At first Canales thought an animal was on the loose, but then she found out the chaos was over some sort of fight near the elephant enclosure.
"This guy from the zoo said someone got attacked," Canales said.
From SKY9, you could see one Denver Police officer running up to a group of a half dozen others already taking control of the suspect.
Several zoo security guards also surrounded the man.
"I saw the cops running by this way and I saw them blocking everything off over there with the yellow tape," Canales said.
"It's unfortunate anytime you have to come to a place where families want to come and you have a situation where police have to be called, that's unfortunate for everybody involved, we don't want to come to a situation like this, and the zoo really doesn't want us to come here," Jackson said.
It all happened around the time when the zoo closed its doors, but zoo-goers were still wandering around because they had until 6 p.m. to exit the zoo.
Police say they have a lot of witnesses to interview.
Police were unclear about the relationship between the man and the woman.
An autopsy will be done on the man to figure out what led to his death. His name was not released.
Police would not confirm whether he was shot with a Taser or if any other restraint methods were used.
(KUSA-TV © 2011 Multimedia Holdings Corporation)