Jason's family wanted the murder conviction, as did Assistant District Attorney Jack O'Neill, who said his office plans to retry Zysk on that charge in November.
"I'm very disappointed. This is a felony of the second degree," O'Neill said of the manslaughter conviction, which carries a maximum sentence of five to 10 years in prison.
"It's what you'd get if you pepper-sprayed somebody in the face. It certainly doesn't hold him accountable for beating a baby to death," O'Neill said.
Defense attorney Evan Hughes said he would challenge the prosecution's retrial plans on grounds that it violated Zysk's constitutional rights not to be tried twice for the same crime.
He told the jury that Zysk should be convicted of manslaughter - not murder - because he never meant to seriously hurt or kill Jason, but only wanted to stop the child from crying.
"He was an exhausted, frustrated parent in the middle of the night who lost control. He will have to pay for that for the rest of his life, but it's not murder," Hughes said after leaving the courtroom.
The day of the incident, Zysk and Jason's mother, Daniela Gonzalez, had used heroin in front of the child and Zysk's daughter.
The defendant's rage was due in part to the fact that the couple had run out of drugs by nightfall, O'Neill told the jury.
Jason's father, Jason Larkin Sr., 28, said he was surprised that the jury deadlocked on the murder count. "I thought it was pretty clear," he said.
"We're very, very disappointed with the outcome. That was a 3-year-old child. He was beat to death. There's no other way to put it," said John Larkin, the victim's grandfather.
Zysk is a recovering heroin addict who was discharged from the Navy because of his addiction. He will be sentenced May 30.
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