PHOENIX — A man arrested in one of the city’s two serial killer investigations pleaded guilty yesterday to assaulting a homeless man with a knife, and prosecutors agreed to drop an attempted murder charge.

Jeff Hausner’s brother, Dale Hausner, is charged with seven counts of murder in the shooting spree that wounded at least 17 people and killed seven, and his alleged accomplice, Samuel Dieteman, is charged in two of the killings.

Investigators said the April 14 stabbing of the homeless man was linked to those serial shootings, but they haven’t specified the link.

Police have said Jeff Hausner, 40, attacked the homeless man after the man approached him and Mr. Dieteman outside a supermarket.

Jeff Hausner now faces a maximum of nearly nine years in prison. His sentencing was set for April 20.

Jeff Hausner’s lawyer, Candice Shoemaker, said the cases were only connected “because of the presence of Mr. Dieteman. Jeff feels he was wrapped into everything that was going on.”

Police said Mr. Dieteman had nothing to do with the stabbing.

Jeff Hausner did not agree to testify in the case against his brother and Mr. Dieteman as part of the plea agreement, prosecutor Vince Imbordino said.

Ms. Shoemaker said her client just wanted the case resolved.

“He didn’t want to go to trial given that the top charge was attempted murder, a charge which carries substantially more [prison] time,” she said.

Authorities spent weeks looking into his involvement after arresting Dale Hausner and Mr. Dieteman last August in the serial killer investigation. Investigators said Dale Hausner and Mr. Dieteman cruised Phoenix area streets late at night in a car, randomly blasting at people with shotguns.

The attacks are thought to have started May 17, 2005, with the fatal shooting of Tony Mendez, who was riding his bicycle at night along a downtown street.