What caused the death of convicted arsonist Michael Marin in a Phoenix courtroom?

PHOENIX - A witness describes the scene after a convicted arsonist apparently swallowed a poison pill moments after learning he was likely headed to prison.

Just seconds after the verdict was announced Michael Marin seemed to slip something into his mouth.

"It was the most shocking thing I've ever seen," said Chris Rapp, a prosecutor who was in the courtroom on Thursday.

Marin was accused of burning down his Phoenix mansion three years ago because he couldn't afford the mortgage.

Just after being convicted of felony arson, facing a maximum 21 years in prison, he apparently committed suicide.

"He ended up on the floor, and his defense attorneys were helping him, and eventually it just got worse and worse and worse," Rapp said.

Marin died before he got to the hospital.

The speculation is that this was a suicide by a toxic substance.

"There are a number of things you can buy in a relatively short period of time that will cause your death," said Brad Garret, former FBI agent. "I wouldn't be surprised with investigation you will be able to determine before the toxicology test what he took."

Police will check Marin's computer and credit card records for clues about what killed him and where he may have gotten it.

It's possible Marin could have been given a lighter sentence -- been out of jail in less than 6 years -- but it appears he didn't want to spend any more time behind bars.

"As a prosecutor of course you want to convict the guilty, but to see someone do that is sad," Rapp said.