“Narcissists rewrite history to escape accountability,” Nichole Schmidt wrote on Twitter on Monday, along with hashtags “justice for Gabby” and “the truth will be revealed”.
Laundrie claimed responsibility for Ms Petito’s death in a note found in a dry bag next to his remains in the Myakkahatchee Creek Environmental Park and released by the Laundrie family attorney last week.
He claimed Ms Petito, 22, was injured after falling while trying to cross a creek and that her death had been “merciful”.
“I couldn’t find her for a moment, shouted her name. I found her breathing heavily gasping my name, she was freezing cold,” Laundrie, 23, wrote.
“When I pulled Gabby out of the water she couldn’t tell me what hurt. She had a small bump on her forehead that eventually got larger.
“Her feet hurt, her wrist hurt but she was freezing, shaking violently, while carrying her she continually made sounds of pain, laying next to her she said little lapsing between violent shakes, gasping in pain, begging for an end to her pain,” he continued.
“I thought it was merciful, that it is what she wanted, but I see now all the mistakes I made.
“I panicked. I was in shock. But from the moment I decided I took away her pain, I knew I couldn’t go on without her.”
Ms Petito’s remains were located in a shallow grave near the Grand Teton National Park in Wyoming last September.
Her death was ruled homicide by manual strangulation by a coroner.
Experts who have travelled to the location have questioned Laundrie’s explanation.
They say the area has cellphone reception and was not far from where the couple’s camper van was parked.
Laundrie died by suicide from a self-inflicted gunshot wound to the head in a reserve five miles from his parent’s home in North Port, Florida.
Ms Petito’s parents Joseph Petito and Ms Schmidt have filed a lawsuit alleging that Chris and Roberta Laundrie not only knew their daughter was dead, but knew the location of her body, as they frantically searched hoping to find her alive.
They appeared for a preliminary hearing at the Sarasota County Circuit Court last week where a judge was deciding whether the case can proceed to trial.
The Petitos allege Brian’s parents released a misleading statement through their lawyer that their daughter was still alive when they knew their son Brian had already killed her.
They also claim the Laundries were helping Brian evade law enforcement.
The Laundries have sought to dismiss the lawsuit, and deny any knowledge of Ms Petito’s killing.
Throughout the hearing, Ms Schmidt rubbed a locket containing her daughter’s ashes.