Kenneth Johnson had confessed that he punched a 14-month-0ld in the stomach after becoming frustrated over the child's crying, police said.

The boy died of internal injuries in Fresno, Calif., according to the coroner's office. His death had been ruled a homicide.

But these apparently were not enough evidence to charge Johnson for the child's death.

The 30-year-old man, who was the boyfriend of the child's babysitter, was released from Fresno County Jail after District Attorney Lisa Smittcamp declined to file charges against him.

"What must be considered in all cases, is that while statements of an accused individual may be used against them, the law requires that there be other evidence of the crime independent of the accused's statement in order to file criminal charges or obtain a conviction," the Fresno County District Attorney's Office said in a statement on Friday.

Johnson was arrested earlier this week after, authorities say, he admitted hurting Jose Luis Espinoza III in November. Detectives obtained the confession following a lengthy interrogation, Fresno Police Chief Jerry Dyer told reporters during a news conference on Wednesday.

Dyer said police were called to an apartment complex in Fresno at about 2:40 p.m. on Nov. 20. The child was found unconscious and was later pronounced dead at an area hospital.

The child's babysitter, a 20-year-old woman who was not named, told detectives that she was cleaning the apartment when the boy's older sister, a 7-year-old, said he wasn't breathing, Dyer told reporters.

Doctors did not find visible signs of injuries. But over the next two days, initial autopsy results revealed that the child had internal trauma, leading the coroner's office to rule the death a homicide, Dyer said.

Detectives honed in on Johnson, who was seen on video surveillance leaving the apartment just before 3 p.m. on Nov. 20. At that time, Johnson also was in violation of a three-year restraining order imposed by the court after an earlier domestic dispute between him and the babysitter.

Johnson admitted being at the apartment complex, but denied hurting the child. Dyer said detectives were not convinced and interrogated him again on Tuesday -- this time for three hours. Johnson confessed and was immediately arrested on a preliminary murder charge, Dyer told reporters.

Johnson was released about two days later after the county district attorney declined to file charges, saying the agency needed more information from the coroner's office.

In the statement, the agency outlined ethical considerations that prosecutors must take into account in filing criminal charges. The district attorney's office said that prosecutors file charges only if there has been a complete and thorough investigation, if there is enough admissible evidence that a crime had actually occurred and a suspect had committed it, and if there is a probability of conviction.

A person's confession is not enough, according to the statement.

"The Fresno County District Attorney's Office remains committed to identifying and prosecuting those individuals who commit crimes on the young and vulnerable in our county," the agency said, declining to comment further on the case.

The child's mother, who lives in the same apartment with the babysitter, was in Modesto, more than 90 miles northwest of Fresno.

In a statement, Dyer said the district attorney has not requested more information from detectives, but has asked for further examination from the coroner's office, the Fresno Bee reported. He said investigators remain confident that Johnson is responsible for the child's death.

Johnson was followed by reporters and cameras as he walked out of jail earlier this week.

"I just apologize for the situation. I just don't know right now," he told ABC affiliate KFSN-TV. "Can I just not comment right now, please?"

A reporter asked why he was at the apartment despite the restraining order against him. Johnson said it was a "peaceful contact."