Courtesy of Dina Shacknai Dina Shacknai and her son, Max, 6. The California mom believes her son's death last summer was no accident.

The mother of 6-year-old Max Shacknai believes her son was slain inside his millionaire father's Coronado, Calif., mansion last summer and wants police to find out who did it.

"He was a victim of assault and a victim of homicide," Dina Shacknai told NBC News on Thursday. "Even though nothing will bring my only child Maxie back, I owe it to him, as his mother, to make sure the true facts of his death are known."

Dina Shacknai is at odds with the conclusion of Coronado police, who have called Max's death a tragic accident. Police say the boy fell over a railing and tumbled down the stairs at his father's historic John D. Spreckels mansion on July 11, 2011. He struck the carpeted floor face first, fracturing his forehead, police said. He was not breathing and did not have a pulse when paramedics found him. Max died a week later at Rady Children's Hospital in San Diego.

Dina Shacknai and the boy's father, Jonah Shacknai, a pharmaceutical company executive and multimillionaire, divorced in 2009, she said. She said they kept their relationship amicable, for the sake of their son.

"We believed in co-parenting and wanted the best for Maxie," Dina Shacknai said.

Timeline of events in the Spreckels mansion deaths

She said Max was visiting his father, spending time at the mansion to be with his two step-siblings, Jonah Shacknai's two children from a separate marriage. The older siblings, however, were not at the mansion at the time of Max's fall, she said.

She said the terrible set of events continue to shock and horrify her to this day.

Max Shacknai

Days after the boy's fall, his father’s girlfriend, Rebecca Zahau, was found hanging nude from a second-story balcony in the same house. The San Diego Sheriff's Department ruled Zahau's July 14 death a suicide.

Shacknai hired two independent experts to review the Coronado police's findings.

“Things just didn’t add up to me," she said. “When I started this process all I knew is that I wanted the truth, wherever that led, like any parent would."

Forensic pathologist Dr. Judy Melinek contributed to the independent investigation, calling the classification of Max’s death "inaccurate."

“It would be more accurate to certify that manner as a homicide, where homicide is defined as death at the hands of another,” Melinek said in the statement.

Melinek said there is the possibility of a scenario in which Max was assaulted, leading to his fall from the balcony. Here's the investigator's take:

"The perpetrator injured [Max's] face and shoulder and Max then was pushed against or backed into the second-story railing, causing the pattern(ed) injuries along his back." "He was then either lifted over the banister or escaped over the banister, falling down to the front entry way..." causing further injury to his back. "He landed on the top of his head and collapsed with his legs following, rather than with his legs first and his face second."

Jonah Shacknai also asked California Attorney General Kama Harris to review investigators' findings that his girlfriend killed herself by tying her wrists and ankles and hanging herself.

Zahau’s family also has called for a separate investigation. According to NBC News, the family appeared on two episodes of the afternoon television show "Dr. Phil" in November to talk about the case.

For more, view NBCSanDiego.com's report on mansion death investigation

A spokeswoman for the Coronado police said the department will review information presented by Dina Shacknai's investigators.

“They provided the information, we told them we would look at it. We don’t even know what we’re looking at at this point,” Lea Corbin, spokeswoman for the Coronado Police Department, told NBC News.

Attempts to contact Jonah Shacknai were unsuccessful on Thursday. A telephone call to his lawyer's office was not returned.

'He was a gift'

A year later, Dina Shacknai said she is still filled with grief.

"He was such a beautiful, generous child and we donated his organs; his liver saved an infant and his kidneys went to two adults," she said.

"He was a gift to us and gift to all of us. I was the luckiest mom in all the universe," said Dina Shacknai as she fought back tears during a telephone interview from Arizona.

She said she misses her son's bright smile, and his voice.

"Every time I saw Maxie, I felt I had won the lottery. He was the best," she said. "It is a terrible thing to now know what happened to someone you love. He deserves justice."

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NBCSanDiego.com contributed to this report.

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