DENVER -- It's called "secondary poisoning" and it occurs when a scavenger, like a bald eagle, feeds on the carcass of a euthanized animal that has not been properly disposed.

Over six weeks, 7NEWS discovered 13 dead animals in the Dumpster outside the Colorado Humane Society on South Platte River Drive.

Through testing, sources, and identification, 7NEWS confirmed at least six of those animals had been euthanized.

According to experts, proper disposal of a euthanized animal includes incineration or burying the animal at least 2 feet in the ground.

"An animal thrown into a Dumpster is not a burial. It's an improper disposal," said Roger Gephart, an investigator with the United States Fish and Wildlife Service. "If an animal has been improperly disposed of and a bird such as an eagle eats the flesh of that animal, sometimes they'll even die right on the flesh of that carcass."

"They're just tossing the dead animals into Dumpsters. That's absolutely appalling. I'm completely shocked," said Peter Reshetniak, president of the Raptor Education Foundation.

CALL7 Investigator Tony Kovaleski asked Reshetniak, "How wrong is this?"

Reshetniak answered, "Somebody should go to jail. That's how wrong this is."

The Colorado Humane Society has now admitted to improperly disposing of two of the 13 animals discovered by 7NEWS. However, that admission came after the executive director, Mary Warren, had denied everything.

Warren insists she knew nothing of the improper disposal of animals.

In a statement to 7NEWS, the Colorado Humane Society said a staff person mistakenly made the decision to dump two euthanized dogs into the Dumpster and that such action would never happen again.

Current and former employees of the Colorado Humane Society told 7NEWS they were ordered by Warren to put dead animals in the Dumpster.

They said it was done to save the $12 per animal it would have cost to have them cremated.

When asked about the accusations, Warren said the whistleblowers lied.

"Something's broken down in the whole system and if it's just to save a buck then maybe these folks should be involved in some other business," said Reshetniak.

In recent years, U.S. Fish and Wildlife has investigated the deaths of more than 140 bald and golden eagles as a result of secondary poisoning.

"It only takes a bite or two of the tissue and they'll die right on the carcass," said Gephart.

U.S. Fish and Wildlife only investigates cases involving protected wildlife, but Gephart said the secondary poisoning can affect any animal that scavenges on carcasses, and wrapping a euthanized animal in a garbage bag is not enough.

"Scavengers will tear apart the plastic bags, whether its a coyote or a bird, they will. Seagulls are notorious. They eat garbage and they will tear open plastic bags to get at what's inside," Gephart said.