Chris Watt's in-laws revealed their heartbreak at being unable to see their granddaughters before they were buried - in case the children's bodies caused an explosion.

Sandy and Frank Rzucek today spoke publicly for the first time since their daughter Shanann Watts, 34, and her two kids Celeste, three, and four-year-old Bella were senselessly killed .

"We got permission from [Watts'] side of the family to have the grandchildren and bury our grandchildren with their mum.

"But the hardest thing was flying them here, because they were in crude oil for four days. So they were flammable. So we couldn't cremate them. They would have blown up a building," tearful Frank said.

Speaking to The Dr Phil show in the US, the heartbroken couple told how the process of removing the girls' bodies from massive oil drums began at 5am on August 16, 2018, and would go on for nearly 14 hours.

The grandparents held hands as they appeared on The Dr Phil show (
Image:
Dr Phil)

But Celeste and Bella's skin came off their bodies when men in self-contained breathing gear removed the corpses from the tanks.

The two 400-barrel tanks were initially drained of the crude oil.

The girls' mum and unborn baby brother Nico were found just 100 yards away in a shallow grave.

Frank continued: "They had to have a bigger coffin and then they had to seal it with a certain wrap, so the gasses wouldn't leak out. That's pretty sad, isn't it? So we three here never got to say good-bye to our family or see them."

His wife Sandy added: "Never got to hug them. Never got to say 'I love you' like most people do."

The children and their mum were finally laid to rest (
Image:
Weld County DA)

The devastating tributes come after troopers revealed details about the daunting retrieval process at Anadarko Petroleum Corp, Colorado.

"While the first crude tank was being off-loaded, Trooper Bandy, Reeder and I put rope handles on two containment pools," Tropper Wilson wrote in a report.

"We anticipated the bodies to be covered in crude oil and were going to use the pools to put each body in, once recovered."

Watts dumped the bodies of Celeste and Bella into oil tanks (
Image:
Weld County DA)
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The troops arrived on the scene at 9am on August 16.

"Sergeant Armstrong went up to the top of the tank and looked in the thief hatch," states the report.

"Sergeant Armstrong told me he could see what looked like a body face down on the south side of the tank."

The victims were then carefully removed from the large tanks (
Image:
Weld County DA)

The victims were then carefully removed from the large tanks.

"During this extraction there was some skin slippage where we had to touch the victim’s body. There was also some skin on the plywood where her back made contact," added the report.

Chris Watts, 33, is serving out his life sentence at the Dodge Correctional Institution in Wapun, Wisconsin.

The monster pleaded guilty to murdering his pregnant wife and their young kids.

Chris Watts is now serving life in jail (
Image:
ABC)
The monster killed Shanann and their two daughters; four-year-old Bella (left) and three-year-old Celeste (right) (
Image:
Instagram)

He cried and shook in court as he listened to Frank Rzucek speak about how his pregnant daughter and granddaughter's lives were "taken by an evil, heartless monster" who "took their bodies out like the trash".

"God only knows what happened that night but life will never be the same. They had all their lives to live.

"They were taken by a heartless one, taken by an evil monster the heartless monster. You took them out like trash - you disgust me," Frank said in court in November.