The O'Shells had one supervised visit with Alyssa, according to Paul Cuin. The baby turned her head away from her parents several times and authorities interpreted that as confirmation of abuse.
Dave O'Shell became a chief suspect when he admitted that he often held her by the legs upside down -- which he said made the baby smile, according to the Post.
Cuin said the signs of SMA were evident in Alyssa, "but no one saw it" until the baby's foster mother took her to the doctor because she was failing to thrive.
A pediatrician at Children's Hospital noticed the classic symptoms: the baby's thumb turned inward, a "bell-shaped" stomach and "frogs legs" that wouldn't straighten, according to Cuin. Alyssa's breathing was labored and she struggled to hold her head up.
Suspicious, the doctor called for genetic tests, but no one alerted Alyssa's parents, according to Cuin.
"If they had had a little bit of hope," Cuin said, "this all would have been different.
On July 9, the results confirmed SMA, and on July 11, a caseworker called the Cuins' lawyer. The O'Shells had been dead nearly two weeks.
By July 16 the Cuins went to court and a judge granted them custody.
The Cuins defend their son-in-law against abuse charges, but are still struggling to understand why he murdered their daughter.
"David was a very stable individual," said Cuin. "It shocked us. But I fully understand the pressures he was under."
Cuin said O'Shell had lost all hope, told by his lawyer that he would go to prison and lose not only his daughter, but his wife, his job and his military status. If arrested on felony abuse, he would have had to raise $50,000 bail.
Two days before the murder-suicide, O'Shell told his wife he was "going to shoot people" so police would have a reason to arrest him, according to the Denver Post. He became increasingly despondent.
One June 30, the couple was scheduled to meet with lawyers and a criminal investigator about the abuse charges. Jackie Cuin tried to call her daughter but got no answer.
She went to check on her at the house, but was too afraid to enter, calling her husband.
Paul Cuin found the bodies: Tiffany, who had been shot in the head twice, was covered in blood in bed. Dave's legs were sticking out the bedroom doorway.
"I haven't forgiven him," said Cuin. "And I don't know if I will ever be able to."
Cuin and his wife now live day-by-day, and their awareness campaign is what keeps them going.
"We don't want the kids' death to be in vain," he said. "We want something good to come of it."
"I don't have a problem at all with social services coming and taking a child and doing an investigation," said Cuin. "There is a need for this service. There are bad people out there and kids need to be protected."
"But the system did the opposite," he said. "It tore a family apart."