It began like a typical call to the emergency services. The operator instructed a woman how to perform chest compressions on her husband, who was unconscious on the floor of their Manhattan apartment.

But six minutes into the call, Shelley Gilbert revealed the uniquely heart-wrenching circumstances she found herself in on the afternoon of 4 January 2015.

“He has been shot,” Ms Gilbert said, trying to keep her voice calm.

When the operator asked her who had done it, she quickly added: “My son, who is nuts. But I had no idea he was this nuts. He shot him in the head.”

Moments earlier she had greeted her son, Thomas Gilbert Jr, and had left him alone with his father, Thomas Gilbert Sr, a fixture on Wall Street who had founded a successful hedge fund.

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The killing stunned New York society four years ago with its scandalous details: an Ivy League-educated child of privilege accused of killing his father for cutting back his $1,000 weekly allowance.

A jury heard the agonising call during the first day of testimony in Thomas Gilbert’s trial on second-degree murder charges.

The 34-year-old, bearded and visibly gaunt, kept his head low as the 11-minute call played for the jury in state Supreme Court in Manhattan. Later on, Justice Melissa Jackson reprimanded him for shouting “objection” several times, urging him to “refrain from these outbursts.”

Thomas Gilbert’s lawyers had argued unsuccessfully that he was unfit to stand trial because of mental illness, but prosecutors worked to paint him as a vindictive child who had planned to kill his father for reducing his weekly allowance by hundreds of dollars.

“Rather than being a compulsive act of a person deranged, the evidence will show that this was a planned and calculated act of a self-absorbed and vengeful son,” Craig Ortner, an assistant district attorney, told jurors.

Mr Ortner said Thomas Gilbert had showed up unannounced that afternoon, and had asked his mother to leave them alone to “talk about business.”

She left, but quickly returned.

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She found her husband on his back, his left hand covering a gun on his chest. Prosecutors have argued that the scene was clumsily staged to look like a suicide.

Shelley Gilbert testified that she had agreed, reluctantly, to cut her son’s allowance in an effort to force him to seek psychiatric help.

“For years we wanted Tommy to go to a psychiatric hospital. We tried everything we could,” she said. “This wasn’t a disciplinary problem. This was a health problem — a severe one.”

Thomas Gilbert Sr was an energetic 70 year old who still ran the successful hedge fund he founded in 2011, Wainscott Capital Partners Fund, when he was killed. He often played tennis at the River Club. He lived with his daughter and wife of 33 years in an apartment on Beekman Place in Turtle Bay.

“Tom Gilbert’s financial success was self-made. He made his money by wisely investing in the stock market,” Mr Ortner said. “At the age of 70, he still had a lot of living up to do.”

Unlike his father, Thomas Gilbert Jr, a graduate of Princeton University, had known few successes outside surfing and was not able to keep a steady job, prosecutors said.

Mr Ortner said the younger man grew dissatisfied with his father’s diminishing financial help, even as he enjoyed a lavish lifestyle that included a Chelsea apartment, fine dining and vacations in Europe and Hawaii.

“He was able to do these things because his parents supported him,” he said.

Thomas Gilbert Jr received psychiatric treatment and took medicine for depression, anxiety and other mental illnesses, “like many other New Yorkers,” Mr Ortner said. Still, he said, he “knew what he was doing.”

Investigators determined that he had tried to order a gun via Facebook, but later drove to pick it up in Ohio after failed attempts to have it delivered to New York, prosecutors said.

“The evidence will not support that he was legally insane,” Mr Ortner said. “The defendant was fully aware of what he was doing. The evidence will show that he executed his father in cold blood.”

But Thomas Gilbert’s lawyer, Arnold Levine, told the jury that the defendant had a 10-year history of psychotic illnesses and had failed to find proper treatment despite visits to many doctors.

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He had been diagnosed with a long list of mental disorders including anxiety, paranoia, depression and obsessive compulsive disorder.

“He believed that people were trying to steal his soul,” Mr Levine said. “Don’t be thrown off by motives manufactured. He was given serious medication for a decade. It renders him criminally not responsible.”

Shelley Gilbert said she was at first encouraged by her son’s impromptu visit.

“He came in and told me he wanted to talk to Dad about business, and so I was excited about that,” she testified. “That could be good for both of them.”

But after leaving, she said, she decided to return and grew worried when she put her ear to the door and heard silence.

“My husband was lying on the floor,” she said. “I remember calling his name and hoping he was just knocked out. And then I looked at him, and then I knew he was no longer with us.”