FRESNO, Calif. (AP) — A Marine charged with beating his 3-month-old son to death is guilty not of murder, but of improperly handling the child, his lawyer said Monday.

Lance Cpl. Robert Quiroz, 20, did not speak or enter a plea during his first appearance in Fresno County Superior Court. He is charged with murder, two counts of assault on a child causing death, and corporeal injury to a child in the death of his son, Roman Quiroz. His arraignment was scheduled for Dec. 11.

Defense lawyer Ernest Kinney said after the hearing that the death was a case of "improper handling," not murder.

"He definitely played a role. The issue is we think it's absolutely not a murder case," Kinney said. "Our position is that it was an accident."

Kinney said Quiroz was feeding the boy Nov. 18 and accidentally bumped his head when placing him on a sofa. After checking on his toddler daughter in another room, he found the infant choking on his own vomit. He tried to give his son CPR and possibly broke two of the boy's ribs, he said.

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Investigators said Roman died of beating injuries after he was found unconscious in the family's Fresno apartment.

Authorities have not disclosed a motive for the slaying, but the Marine told a TV station in September that he felt overwhelmed as a single parent of two and struggled to accept his son.

Quiroz was called home in August, three days after being deployed to a training camp in Kuwait, after his wife gave birth to the boy but suffered an aneurysm shortly after. He arrived in Fresno faced with ending her life support and assuming sole responsibility for their two children.

A state agency also is looking into whether Fresno County social workers failed to investigate whether abuse was happening after the infant suffered a broken arm in October.

Quiroz is being held at the Fresno County jail in lieu of $2.3 million bail.