Opening statements begin on Monday afternoon in the murder trial of a Florida dad who admitted to throwing his daughter off a bridge four years ago.

A jury of nine men and seven women will hear testimony that John Jonchuck dropped his 5-year-old daughter, Phoebe, to her death. Johnchuck has pleaded not guilty by reason of insanity.

Phoebe’s body was recovered from Tampa Bay on Jan. 8, 2015, about an hour after an off-duty police officer heading home said he saw Jonchuck’s white PT Cruiser stop on the Dick Misener Bridge in St. Petersburg.

Jonchuck then “reached into the back seat and grabbed the girl,” police spokesman Mike Puetz told PEOPLE at the time. “Then he threw her over the railing and into the water.”

The officer says he heard a splash as the child fell into the bay below. He then radioed for help and climbed over the railing himself, hoping he could rescue the child. But “he couldn’t see her, because the current is extremely strong there,” said Puetz.

Both the prosecution and the defense acknowledge that Jonchuck, 29, killed his daughter, but the defense claims that he was mentally ill at the time. They argue that he should be found not guilty by reason of insanity.

Court filings obtained by PEOPLE allege that Jonchuck said he heard “voices” that told him to kill Phoebe. During interviews with a mental health doctor, Jonchuck said, “I was hearing voices saying that if me and Phoebe didn’t die, everybody was going to go to hell.”

RELATED: Florida Father Accused of Throwing 5-Year-Old Daughter Off Bridge

Jonchuck’s public defender hoped to have the trial postponed, but a judge ruled him competent to stand trial on charges of first-degree murder. Jonchuck has been in a state mental hospital since Phoebe’s death.

RELATED: John Jonchuck, Accused of Throwing Daughter off Bridge, Is Competent to Stand Trial: Judge

Just hours before Phoebe’s death, police had questioned Jonchuck for acting erratically. His attorney at the time — with whom Jonchuck had been meeting in reference to a custody dispute, who is distinct from his public defender — had called 911 after observing Jonchuck’s behavior.

The Department of Children and Families hotline also had received a call voicing concerns that Jonchuck was unstable. DCF placed a notation about the call in a file for a caseworker to handle at a later date. Twelve hours later, Phoebe was dead.

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During his first interview with detectives after the incident, Jonchuck said, “My name is God and you shall address me as such.” His mother told cops that her son struggled with schizophrenia and bipolar disorder. He had not been taking his medicine.

Jonchuck also told the psychiatrist that he has “bad dreams” about his daughter in which he couldn’t “get her to wake up.”

He said of himself: “I was a really good father and she loved me so much and I always promised her that I’m not going to let anything happen, and I did.”

The trial is expected to last at least a week.