DARBY TWP., Pennsylvania (WPVI) -- A 30-year-old man has been charged with first-degree murder in the fatal weekend fire at a home in Delaware County, Pennsylvania.
Officials say it was intentionally set by Aaron Clark, who was taken into custody about six hours after the fire started.
Clark also faces three charges of attempted murder.
"Aaron Clark has committed one of the most evil and vile acts I have ever seen," Delaware County District Attorney Jack Stollsteimer said.
Stollsteimer and other officials detailed the case during a Wednesday morning news conference.
Stollsteimer says Clark set the fire in retaliation against his ex-girlfriend who was trying to end her relationship with him.
Officials say Clark worked for the postal service in Philadelphia, where he met his former girlfriend who was a coworker.
Investigators say when she tried to end their relationship, he retaliated by making threats to do "something big."
Officials say she reported the threats to her employer and police, but officers hadn't had a chance to make contact with Clark before the fire.
The fire broke out at the home on the 600 block of Sharon Avenue just after midnight Sunday.
Officials say 20-year-old Olivia Drasher, who had cerebral palsy, was wheelchair-bound and non-verbal, could not escape the flames.
She is the younger sister of Clark's ex-girlfriend, who was not home at the time.
"She was killed by smoke inhalation and her body literally being burnt to death," Stollsteimer said. "You can imagine, on a December evening, as she's sitting in this household dying because of the smoke and because of the fire that is burning out her body. I have never seen somebody do something as heinous as Mr. Clark did here."
According to a GoFundMe page, Drasher's mother, who is a Philadelphia police officer, and Drasher's twin were able to escape unharmed. Drasher's nurse escaped with injuries.
Firefighters had to leave the home twice due to the intensity of the blaze.
Stollsteimer and Darby Township Police Chief Mike Sousa both say this was the most evil act they have seen in their careers.
Sousa called Clark "an absolute monster."
He says authorities have knowledge that Clark knew who was in the house at that time of day.
"He couldn't kill the woman who was trying to break up with him so he took it out on her paraplegic sister by intentionally torching the house when he knew the family was there," Stollsteimer said.
Stollsteimer says Clark did not stop his horrific ways even while in custody.
"Even after he set this fire and intentionally killed her sister, he was in police custody and sending more terroristic threatening messages to her just to further terrorize her, after having killed her sister and attempted to wipe out her entire family," Stollsteimer said.
Officials say Clark was sending those threatening messages while in custody from an Apple Watch that he hid inside his body when he was arrested.
When police confronted him about how he was sending those threatening messages, officials say he assaulted a police officer.
He is facing additional charges for this incident.