Dale Hausner, 40, was pronounced dead at a hospital near the prison in Florence, The Arizona Republic reported. The Department of Corrections said the cause of death was still unknown.
Hausner, who received six death sentences in 2009, had attempted suicide at least once before his trial. While he did not admit being a serial killer, he said he wanted to be executed as quickly as possible.
In 2005 and 2006, Hausner and Samuel Dieteman, who shared his Mesa apartment, went on a series of shooting sprees that left eight people dead and 19 wounded. At least 10 animals were also killed. Dieteman is now serving a life sentence.
The two became known as the "Serial Shooters."
Randy Hausner sent the Republic an e-mail about his brother's death.
"We as a family stand with the victims of the crimes and their families," he said. "Today, a murderer died and is now going on to face the Ultimate Judge. Again, our thoughts and prayers are with the innocent people that were victims of his senseless crimes."
The Arizona Supreme Court upheld the death sentences last year.