The Common Pleas Court jury of seven women and five men had deliberated about 21 hours since Thursday and said Monday it was deadlocked on third-degree murder.
Judge Linda Carpenter set sentencing for May 30 and, as expected, ordered Zysk, who had been free on house arrest, taken into custody. Zysk faces five to 10 years in prison for involuntary manslaughter. He would have faced 20 to 40 years had he been found guilty of third-degree murder.
Assistant District Attorney John O'Neill said he would retry Zysk on the murder charge, a move the defense says it will oppose.
Hughes said he believed a retrial would violate the Constitution's "double jeopardy" clause barring multiple trials for the same crime.
O'Neill said his office had conducted similar retrials "any number of times."
The enmity between Zysk's family and that of ex-girlfriend Danieala Gonzalez, 24, the mother of the slain Jason Larkin Jr., was thick in the courtroom. Carpenter repeatedly warned both sides against any outbursts, and they left separately without comment.
Hughes said he hopes to persuade Carpenter to sentence Zysk to less than the 5- to 10-year statutory term.
"He has lived incident-free since his arrest, and he was never arrested before this case," Hughes said.
O'Neill would not say what sentence he would seek, but he added, "I'm really disappointed by this."
"Involuntary manslaughter is a second-degree felony, the same as pepper-spraying someone in the face, an aggravated assault," O'Neill said. "This is beating a baby to death. How can you say that the death of a little boy who is supposed to be starting his first day of school is the same as pepper-spraying someone in the face?"
The beating happened Jan. 15, 2011, in Zysk's mother's Roxborough home, where Zysk then lived. That night, Gonzalez and Jason were staying over.
Zysk said he punched Jason two or three times in the side between 2 and 3 a.m. because the child would not stop crying, and Zysk was frustrated and sleepless after using heroin earlier.
An autopsy showed the blows pushed Jason's liver against his spine, cutting the organ and causing him to bleed to death.