The bag also contained an expended .410-gauge shotgun shell and a piece of paper referring to serial violence, according to a probable cause statement released Friday night.
The trash bag had been tossed into a bin at the suburban Mesa apartment complex where Dale S. Hausner and Samuel John Dieteman lived, the report said. The men, accused of shooting two dozen people, including six fatally, were arrested late Thursday after police tailed them for a week.
A person who called police said Dieteman drove around selecting random targets. It was only later that police connected Hausner to Dieteman. Police watched them as they "suspiciously drove through the areas of prior attacks and slowing in the areas of vagrant activity," according to the report.
Hausner and Dieteman would take turns driving and shooting, and Dieteman admitted to carrying out many of the shootings, the report said.
It said detectives found a .410-gauge shotgun and ammunition in the suspects' possession. Police also found other guns and long rifles and news clippings about the killings.
Hausner, 33, and Dieteman, 30, were booked Friday afternoon for investigation of two counts of first-degree murder in the Sunday killing of Robin Blasnek and the May 2 killing of Claudia Gutierrez-Cruz and for investigation of 13 counts of attempted first-degree murder. Police said other charges are possible.
At an initial court appearance Friday night, both men were ordered held without bond. Their preliminary hearings were set for Aug. 14 and the court assigned them attorneys.
At a news conference Friday, authorities they had the right suspects.
"These are the two monsters we have been hunting, and I promise you and our colleagues promise you, we are not finished," Phoenix Mayor Phil Gordon said.
The attacks, which began in May 2005, were all the more frightening because another, apparently unrelated serial killer has been preying on Phoenix-area victims at the same time. Even horses and dogs were among the victims.
Assistant Police Chief Kevin Robinson said the shootings attributed to the "Serial Shooter" appear unrelated to the still-unsolved "Baseline Killer" cases: eight slayings and 11 rapes since August 2005. Investigators are scouring the city's trove of unsolved crimes for links to that serial predator.
The cases doubled the fear and paranoia gripping Phoenix neighborhoods in recent months and led law enforcement agencies to devote more than 200 investigators to track down the killers. Authorities said they will move investigators from the Serial Shooter case to the Baseline Killer one.
Robinson described the Serial Shooter suspects as friends who had no obvious connections to any of the victims.
"The best we could tell, they were just random victims. These individuals just picked victims out and that was it," he said.
Police would not discuss what they thought were the men's motives.
They said they knew nothing about the suspects' occupations, but the city said Hausner worked as a janitor at the Phoenix airport and neighbors said he also worked as a freelance photographer.
According to a report in Saturday's editions of The Free Press, of Mankato, Minnesota, Dieteman had lived in at least six different addresses in south-central Minnesota and had dozens of run-ins with local police.
Records obtained by the newspaper show that from 1992 to 1999, police had nearly 40 contacts with Dieteman, including drunken driving incidents, thefts and assaults. The newspaper also found court records showing Dieteman failed to pay child support to his ex-wife. He left Minnesota in 1999.
The most recent shooting occurred Sunday in Mesa, less than three miles from the men's apartment. Blasnek was killed as she was walking from her parents' home to her boyfriend's house.
Hausner's brother, Randy, told The Associated Press his family is devastated by the arrest.
"I mean, who would do something like that?" Randy Hausner said. "That's harming innocent people."