Frank Rzucek, whose daughter and grandchildren were killed by son-in-law Chris Watts, says "heartless" bullies are trolling his grieving family online.
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"There's a lot of ugly out there," he said.
Chris Watts is serving life in prison for the August murders of his pregnant wife, Shanann; his 4-year-old daughter, Bella; and his 3-year-old daughter, Celeste.
"For the past 11 months, piled on top of pain and the grieving of this devastating loss, our family has been subject to horrible, cruel abuse, outright bullying, on a daily basis," Rzucek told reporters Monday in front of the Watts' former Colorado home.
"I don't want to draw more attention to the viral material that has been posted online," he said, "but I will say that our family, including Shanann and her children, our grandchildren, have been ridiculed, demeaned, slandered, mocked in the most vicious ways you can imagine."
Rzucek alleges that the attacks includes threats, fake Facebook accounts and a "constant stream of ugly, evil insults."
"Every time we turn around there's someone trying to capitalize on our tragedy by spreading false rumors and outright lies about Shanann and our grandchildren. We have been subject to threats on our lives all because we are victims in painful tragedy," he said. "It is cruel, it is heartless."
Rzucek said he's reported the abuse to social media platforms but there's been no change.
He had a message for social media companies: "We are calling on you again to do the right thing, to take some responsibility for your platforms and stop this despicable conduct."
And to the trolls, Rzucek pleaded, "please stop."
Rzucek acknowledged that his family isn't the first to be victims of online bullying.
Families of those killed at Sandy Hook Elementary School in Connecticut in 2012 have for years been the target of conspiracy theorists, online harassers and those pushing unfounded claims about the tragedy.
"Families like ours should have the right to be safe ... the right to mourn in peace," Rzucek said. "To Congress or to any person in position of power, we are calling on you to do something to pass laws that will protect victims of unspeakable crimes from this kind of abuse."
In August, right after Shanann Watts, Bella and Celeste disappeared, Chris Watts lied to reporters, saying his family went missing.
"When I came home and then walked in the house, nothing. Vanished," he told ABC Denver affiliate KMGH. "My kids are my life."
Within days, Chris Watts was arrested and the bodies of his wife and children were found.
Chris Watts pleaded guilty to all charges against him, and in exchange, prosecutors did not pursue the death penalty. Chris Watts was sentenced in November to life without parole.
He told investigators he strangled his wife at their home then put his children into the car -- along with his wife's body -- and drove to the oil site where he worked.
There Chris Watts said he strangled both children with the 3-year-old's blanket.
He confessed to burying his wife's body and ditching his children's bodies in oil tanks.
At the time of her death Shanann Watts was pregnant with a boy she planned to name Nico.