The four-year-old daughter of murderer Christopher Watts knew her mother and younger sister had just been killed - and pleaded in vain for her life to avoid being next.

Watts, the Colorado man serving a life sentence for killing his family, drove his two daughters for 45 minutes to an isolated oil field along with his pregnant wife’s dead body, where he then suffocated the youngest, Celeste, with her favourite blanket, and then murdered her sister Bella, according to the attorney of the wife’s family.

Steven Lambert, the family attorney of Shanann Watts, revealed new details about the murder on Tuesday’s Dr. Phil show and said that Bella begged for her life after watching Watts kill her mother and sister on August 13.

“Please daddy, don’t do to me what you just did to Cece,” Bella said.

Prior to her death, Bella stumbled upon Watts wrapping her pregnant mother’s body in a sheet after strangling her to death.

“What are you doing with mommy?” the four-year-old girl asked her father.

Getting away with murder: Britain's most notorious unsolved crimes Show all 3 1 /3 Getting away with murder: Britain's most notorious unsolved crimes Getting away with murder: Britain's most notorious unsolved crimes Schoolgirl Milly Dowler was abducted as she walked home PA Getting away with murder: Britain's most notorious unsolved crimes Alistair Wilson was shot dead at his home in 2004. But despite many leads, police still haven't found his killer PA Getting away with murder: Britain's most notorious unsolved crimes The convicted paedophile Andrew Cunningham was killed last year PA

In the Dr. Phil interview, Mr Lambert said the couple got into an altercation after Watts confessed to having an affair. He also declared their relationship to be over and said he wanted a divorce.

“[Shanann] had said something to the effect of, ‘Well, you’re not gonna see the kids again.’ As a consequence of that conversation, he strangled her to death,” Mr Lambert said.

The attorney said that Bella, who was quite bright for her age, knew “something was likely up.” While Watts drove the two girls, both alive, to the oil field with their dead mother in the truck, Bella urged her father to take her mother to the hospital.

After Bella witnessed her father kill her sister, Celeste, aged three, she attempted to run away before pleading for her life.

“At this point, Bella had unbuckled herself from the vehicle,” Mr Lambert said.

Support free-thinking journalism and attend Independent events

During Watts’ sentencing, prosecutors revealed Bella fought back before he killed her. The prosecutors also argued that he did not kill Bella in rage, but “deliberately and viciously.”

Watts disposed the girls’ body in an oil tank. He buried his wife in a shallow grave.