Josiah Brown died nearly a week after he was pulled from the water.
His aunt, 34-year-old Victoria Moreno, has been charged with his murder.
She initially told responding officers she was only a witness to the drowning, then claimed she was holding the boy's shirt and let him fall into the water, according to a source with knowledge of the investigation. However, surveillance video shows a woman pushing him toward the edge of the pier and tossing him into the water, the source said.
A witness said they noticed the boy running around seemingly unsupervised just before the incident. Other witnesses on the scene described it as an odd situation, with the aunt just standing by while the child was in the water.
Police said Moreno did not attempt to rescue the boy.
Her attorneys said she's been diagnosed with depression, anxiety and bipolar disorder.
The young boy was described as "just a bundle of joy" who is "full of energy, wants to do everything" and "loves his dad."