Phoenix ? One of two suspects charged in a series of random shootings that killed seven people in the Phoenix area pleaded guilty Friday to two counts of first-degree murder.

Samuel Dieteman also agreed to testify against his co-defendant, a move that may keep him from being executed.

Authorities say Dieteman, 32, and his former roommate Dale Hausner, 35, randomly shot and killed seven people, wounded 17 more and killed several animals during a rampage that continued for several months over 2005 and 2006. It was one of two serial murder cases that sent fear through Phoenix and its surrounding suburbs at the same time.

Hausner has pleaded not guilty to seven murder counts and faces the death penalty if convicted.

Dieteman, who also pleaded guilty to one count of conspiracy to commit murder, will be sentenced to death or to life in prison without parole.

The plea agreement “ensures that Samuel Dieteman never again will walk the streets a free man,” Maricopa County Attorney Andrew Thomas said.

According to the plea agreement, the penalty phase for Dieteman’s case won’t be held until after Hausner’s trial, which now is scheduled to begin in early September. The agreement says Dieteman’s testimony will be considered a mitigating factor when his own sentence is decided.

“The agreement between our client, Samuel Dieteman, and the state of Arizona was made in good faith with all parties of the agreement acting in the interest of justice,” said Robert Briney, the Maricopa County legal defender.

Defense lawyer Maria Schaffer said that during his plea, Dieteman said he fatally shot Claudia Gutierrez-Cruz, 20, in Scottsdale on May 2, 2006, and assisted in the shooting of Robin Blasnek, 22, on July 30, 2006, as she walked from her parents’ home to her boyfriend’s house in Mesa.

Gutierrez-Cruz had been walking home from a bus stop when she was wounded. A passer-by found her crawling in the street saying “Ayuda me,” Spanish for “Help me.”

As part of the plea deal, prosecutors dropped 50 criminal counts, including arson and attempted murder charges.