The body of Rudolph Zurick, 40, was found at his home in New Jersey, said attorney Michael J. Mitzner. Zurick had not been charged in the Dec. 15 break from the Union County jail and had been cooperating with the investigation, Mitzner said.
"Everything I understand, he did nothing wrong," said Mitzner, who spoke to Zurick on Monday. "It's hard to know what goes through someone's head."
The death is being investigated, Union County prosecutor Theodore J. Romankow said in a statement.
"This is not a time for speculation, but a time for mourning," he said.
Zurick was scheduled to be interviewed Wednesday about the daring break by inmates Otis Blunt, 32, and Jose Espinosa, 20. Both remained at large Wednesday.
The two used photos of bikini-clad women to hide holes they dug through the cinderblock walls of their adjoining cells in a high-security unit, authorities said. They jumped onto a lower roof, then made it over a 25-foot-high (7.6-meter-high) fence topped with razor wire.
The inmates left behind a thank-you note, signed with a smiley face, that named Zurick, thanking him for the tools they used a thick piece of wire and a 10-pound (4.5-kilo) steel water shut-off wheel.
"You're a real pal! Happy Holidays," said the note, which also included a drawing of a hand with an upraised middle finger.
The note, Mitzner said, was "definitely sarcastic."
"There was no way he gave them any help. He was the one who had noticed they were missing."
Blunt was awaiting trial for robbery and weapons offenses. Espinosa was awaiting sentencing after pleading guilty to manslaughter in a 2005 drive-by shooting.
Authorities are reviewing security measures and have barred inmates from putting pictures cut from magazines on their cell walls.