Maricopa County Sheriff Joe Arpaio said at press conference Tuesday that investigators believe Marin made the cyanide powder found in the vehicle into capsules, local station KNXV reports.
Arpaio says Marin ordered the canister of cyanide off the internet in 2011, before his trial even began.
Court officials said the judge and lawyers were discussing aggravating factors and the jury was out of the room when 53-year-old Marin's face suddenly turned red and he collapsed to the floor.
He was taken to the hospital where he was pronounced dead, according to the Maricopa County Sheriff's Office.
Investigators said Marin deliberately burned down his $3.5 million suburban Phoenix mansion in July 2009 after it failed to sell at a charity auction and he could no longer pay the mortgage. He was seen escaping the fire by climbing down a rope ladder while wearing scuba gear.
A Maricopa County Superior Court jury found Marin guilty last week of arson of an occupied structure. Prosecutors said he could have faced 16 years in prison for the charges.
A video recording of Marin's last moments in court had some officials wonder whether his death was a suicide. The video showed Marin cover his mouth and his hands while hearing the guilty verdict and apparently swallowing something, KPHO-TV reports.
He was also seen occasionally sipping from a sports drink bottle before his body twisted into violent convulsions.
Investigators say they are still waiting for the final toxicology reports to come back.
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