Maricopa County Sheriff Joe Arpaio said the discovery was made Monday, more than a week after Michael Marin's death June 28, The Arizona Republic reported.
A family member called police to report receiving a delayed e-mail sent by Marin the day before his death, saying the car was in Mesa, Ariz., outside Phoenix. Sheriff's deputies found a canister labeled "silver cyanide" that Marin appears to have bought online last year.
Marin, 54, faced a lengthy prison term for burning down his home in the Biltmore Estates section of Phoenix in 2009. He was no longer able to make his mortgage payments.
Footage from a pool TV camera in the courtroom showed Marin putting something in his mouth immediately after the verdict. He collapsed in convulsions a few minutes later.
The discovery of the cyanide appears to be additional evidence that Marin committed suicide, the newspaper said.