Michael Marin was found guilty of deliberately burning down his $3.5m (£2.2m) Phoenix, Arizona, mansion.
Now Maricopa County sheriff's deputies are investigating whether the 53-year-old put something in his mouth that led to his death after the verdict was read out.
"We will have to wait until we get toxicology reports from the medical examiner until we can confirm if he did and what the substance was," sheriff's spokesman Jeff Sprong said in a statement.
When Marin's trial began in May, prosecutors said he faced up to 16 years in prison if convicted.
Video from inside the courtroom showed Marin putting his hands over his eyes after the verdict was read and then covering his mouth with both hands.
As Judge Bruce Cohen and lawyers were holding discussions and the jury was out of the courtroom, Marin's face suddenly turned red and he began to make throat-clearing noises before he collapsed to the floor.
Marin's attorneys tried to assist him and paramedics were called. They performed chest compressions but he was pronounced dead in hospital.
Prosecutors claimed Marin set fire to his mansion in July 2009 after failing to sell it at a charity auction.
The court heard Marin could not afford the monthly mortgage of $17,500 (£11,150) and was due to make a payment of $2.3m.
Authorities said Marin escaped the fire by climbing down a rope ladder while wearing a scuba tank and diving mask to protect himself from the smoke.
Marin was a former attorney, banker and art collector who had nearly $900,000 (£573,000) in the bank in 2008 but only $50 (£32) at the time of the fire, a newspaper reported.
