SAN DIEGO -- The animal rights organization, In Defense of Animals (IDA), is calling on the U.S. Department of Agriculture (USDA) to investigate the circumstances surrounding Thursday's death of a 21-year-old African elephant at the San Diego Zoo's Safari Park.

Zookeepers found Umoya lying down in the elephant exhibit shortly before the park opened on Thursday, according to the Christina Simmons of San Diego Zoo Global.

No official cause of death has been released. Veterinary staff was conducting a post-mortem examination to discover a cause of death, but results might not be known for weeks, Simmons said.

"All the information we have and the evidence from the postmortem exams suggests that there may have been some kind of pushing and shoving, and perhaps even a fall," said Simmons.

"There were few injuries that indicated there may have been some sort of aggressive interaction with another elephant," zoo spokesperson Yadira Galindo told 10News' media partner the San Diego Union-Tribune.

Umoya, whose name means spirit, was one of seven African elephants born in South Africa, which were taken to Swaziland in 1994. They were going to be killed, but were brought to the San Diego Zoo Safari Park in 2003. Now, their family has grown to 17, and they mourned the loss much like people.

"They gathered around her and kind of touched her and pushed at her a little bit, and once they confirmed that she had passed on, they all slowly moved back into the other area of the habitat," Simmons explained.

Some animals toss their young aside, but this herd won't let anything happen to Umoya's calves, Kami and Emanti. In fact, their aunts have continued to care for them just like their mother would.

"[They] appear to be doing alright," Simmons said. "They've been hanging out with their aunties. The keepers plan to monitor Emanti closely."

Four-year-old Kami was already weaned, but Emanti is only 18 months. Four other females are lactating and may end up nursing Emanti.

"It's important to remember that these are wild animals with wild behaviors, and also that they're very big strong animals. At 6,600 pounds, small movements can do a lot of damage," said Simmons.

"This is an interaction that would happen normally in the wild. These are social animals that have a strict hierarchy," Simmons added.

IDA's complaint to the USDA states that an adult male elephant, Mabhulane, who is known to throw his weight around was confined with the females and their offspring.

Zoos typically keep males and females separated because the powerful males can injure the females. In the wild, adult males do not live with the matriarchal family groups, the complaint said.

"This elephant's shocking death is another tragic example of how elephants suffer in inadequate and artificial zoo exhibits," IDA Elephant Campaign Director Catherine Doyle said. "There is nothing natural about the San Diego Zoo Safari Park's elephant exhibit or keeping 18 elephants crammed into a space of less than six acres when their natural home ranges can measure hundreds of square miles."

Three elephants have been killed by other elephants in zoos throughout North America since 2001, according to IDA.

IDA is also asking the USDA to investigate why no staff or security personnel were aware of Umoya's fatal injury until trainers arrived in the morning.

Umoya was known to be one of the most dominant in the herd.

"She was definitely high-spirited, a spitfire and appeared to let the other animals know what she needed and wanted," said Simmons. "We definitely have seen that energy in some of her calves, and that energy is living on I have no doubt."