DENVER -- Over six weeks, CALL7 Investigators discovered more than a dozen dead animals in a Dumpster outside the Colorado Humane Society.

Officials with the Colorado Humane Society originally insisted they had done nothing wrong and in an interview with 7NEWS, Executive Director Mary Warren claimed to have no knowledge of any dead animals in the Dumpster.

"So, can I be guaranteed that every time you euthanize an animal, you cremate it?" CALL7 Investigator Tony Kovaleski asked Warren.

Warren replied, "Right. Yes."

"So euthanized animals, dogs and cats, are never thrown in the Dumpster at Colorado Humane?" Kovaleski asked.

"Correct," Warren said.

What she said and what 7NEWS discovered don't match.

On July 24, CALL7 Investigators found two dead animals inside black trash bags in the Dumpster outside the Colorado Humane Society.

It was a fox and a raccoon.

On Aug. 8, 7NEWS discovered three more animals including a cat.

7NEWS hired an independent lab to test liver and fluid samples from two of the deceased animals.

"The results we found would be consistent with what you would expect if an animal had been euthanized," said Petra Hartmann with Industrial Laboratories.

Animals are euthanized with a drug combination commonly known as Beuthanasia. The packaging includes warnings reading, "This product is toxic to wildlife. Birds and mammals feeding on treated animals may be killed."

The warning goes on to read, "Euthanized animals must be properly disposed of by deep burial, incineration, or other method in compliance with state and local laws."

The drug can kill other animals if those animals find a carcass in the Dumpster or in a landfill.

7NEWS also found animals wrapped in trash bags and discarded in the Dumpster on Aug. 20 and Sept. 1.

The most disturbing discovery came on Sept. 7.

7NEWS found six dead animals -- three dogs, two cats and a raccoon -- in the Dumpster.

One of the dogs, an Elk Hound named Sinbad, had belonged to Jerry Mason.

The dog had become aggressive with one of Mason's children and he agreed Sinbad should be "put down."

"He was my best friend," Mason told 7NEWS.

He never expected the Colorado Humane Society would throw the dog's carcass into a Dumpster.

"If I would've known this before that dog went there, that dog never would have ended there," Mason said. "I just hope all animal shelters ain't like this."

A second dead dog, discovered on Sept. 7, was a Rottweiler.

Its former owner did not want to be identified on camera, but told 7NEWS he had paid the Colorado Humane Society to euthanize and cremate his dog.

"Did they give you any indication it was going to be in the Dumpster?" Kovaleski asked the Rottweiler's former owner.

He replied, "No. Not at all."

Ultimately, 7NEWS uncovered more than a dozen animals discarded in the Dumpster outside the Colorado Humane Society over a six-week period.

Testing, identification and sources confirm at least six of those animals had been euthanized.

"I'm horrified. I'm absolutely horrified. It just makes me sick," said Gina Godfrey, a frequent donor to the Colorado Humane Society. "I think what they're doing is wrong, and I think almost everybody would say what they're doing is wrong. And it shouldn't be allowed."

"Have you been told to discard dead animals in the Dumpster?" Kovaleski asked current and former employees of the Colorado Humane Society whom we are identifying as "whistleblowers."

All replied, "Yes."

When asked by whom, one whistleblower told 7NEWS, "Mary Warren, our executive director."

A second whistleblower said, "When an animal is euthanized and there's nowhere to put it, the first person people go to is Mary."

"And what are the instructions?" Kovaleski asked.

"To throw it in the Dumpster," the whistleblower replied.

Kovaleski showed Warren the video of the dead animals discovered in the Dumpster and asked, "You want to tell me the truth now?"

"I am telling you the truth. Absolutely telling you the truth... I knew nothing about this," Warren replied.

"What if Mary Warren says she never told an employee to throw a dead animal in that Dumpster? Would that be the truth?" Kovaleski asked a whistleblower.

"No," said the whistleblower.

"Three employees have said that you have directed them to dump dead animals, sometimes cats and dogs into that Dumpster," Kovaleski said to Warren.

"Not true. Not true," Warren replied.

When asked if the whistleblowers were lying, Warren said, "Absolutely."

Whistleblowers said for each animal left in the Dumpster, the Colorado Humane Society saved $12 on pickup and cremation costs.

After learning of our investigation, the shelter's director of development, Bob Warren, admitted in an e-mail that "euthanized animals might have been mistakenly tossed into the Dumpster."

In an e-mail received Tuesday evening, newly named Colorado Humane Society president Madeline Duncan said, "A member of our staff made the decision to place the bodies of these two animals in our commercial trash bin. This was a mistake in judgment. We accept responsibility for the mistake. It will not be repeated."

The prepared statement failed to address the other 11 animals discovered by the CALL7 Investigators.

The e-mail did include this statement, "The KMGH story has pointed out our shortcomings, but fails to note the immeasurable amount of good our organization has done."

In a conversation with 7NEWS, Waste Management said, "Commercial dumping of dead animals is against state regulation if the animals have not been rendered non-infectious. Typically, the way they are rendered non-infectious is by incineration."

Mary and Bob Warren and the Colorado Humane Society have threatened to file a lawsuit pertaining to the 7NEWS investigation, even threatening to sue individuals they believe, but did not confirm, were talking to 7NEWS.

The Colorado Humane Society has contracts with several municipalities to house and, if necessary, dispose of animals.

Two of those contracts are with Arapahoe County and Littleton.

After learning of the 7NEWS investigation Arapahoe County sent a statement saying, "Arapahoe County was not aware of the allegations against the shelter until your news story aired. We are concerned and we intend to follow up with the Colorado Humane Society to ensure that the animals we drop off are properly disposed of, and that they are properly following the provisions of our contract."

The city manager of Littleton, Jim Woods, told 7NEWS, "The questions that you have raised are a concern to us and we will definitely be reviewing our contract and our relationship with the Colorado Humane Society."

Those who use their time, whether volunteer or paid employees, to help homeless animals are worthy of deep praise and admiration. Their job is not an easy one and on many days, heartbreaking.

The mission of the Colorado Humane Society is also deserving of praise and admiration, however, current and former employees tell 7NEWS that they believe the management at Colorado Humane has lost sight of that mission.

Important Note: The Colorado Humane Society does not have a direct association with the National Humane Society or the Humane Society of the United States. In fact, Colorado Humane Society is not linked to any other animal shelter in the state or nation. It is a private non-profit entity.