Officials in Phoenix said on Friday that they had arrested two men in connection with a yearlong series of shootings that killed six people, wounded 18 more and terrified the metropolitan area.
The two were taken into custody on Thursday night in an apartment in the Phoenix suburb of Mesa, the police said, and put up no resistance.
"These are the monsters we've been hunting," Mayor Phil Gordon of Phoenix said at a televised news conference at City Hall.
The men were identified by Chief Jack Harris as Dale Hausner, 33, and Samuel Dieteman, 31, described by officers as friends.
The police said that the men were still being questioned and that no motive had been discovered for the shootings, which began in May last year and were directed at people walking alone or riding bicycles.
The series of shootings had been connected by the police to the work of an assailant they called the Serial Shooter.
The police said Hausner and Dieteman were identified last Monday, based on tips from the public, forensic evidence and other sources. They have been under observation since then, the police said.
William Newell of the federal Bureau of Alcohol, Tobacco, Fire-arms and Explosives said one of the key connections came after the two men were identified as suspects in arsons at two Wal-Mart stores in the Phoenix suburb of Glendale on June 7. The authorities had surveillance video of the men and their car, and the investigation eventually tied the men to the shootings, Newell said.
The first arrest took place when SWAT team members surprised Dieteman as he left the apartment to take out trash, said Bill Louis, an assistant chief of police.
"He went to the dumpster, the common dumpster in the apartments, and we had a surprise for him," Newell said.
Hausner was arrested when he came out looking for Dieteman.
"We believe he was probably wondering what happened to his roommate," Louis said. "He surfaced and we had another surprise for him."
Hours after the arrests, the police and agents of the firearms bureau searched the apartment and were seen removing rifles and clear plastic bags that appeared to contain bedding and other materials.
They also towed a silver, four-door Toyota Camry that matched the general description of a light-colored sedan that witnesses had linked to the shootings.
A neighbor of the men told reporters that both men had young children who occasionally visited them.
Hausner worked as a photographer who specialized in taking boxing photos. His Web site displayed photographs of him with the boxers Bernard Hopkins and Mike Tyson, among others.
He also shot photos of weddings, birthdays and families, according to the Web site.
Louis described the shootings as "a very slick operation," but declined to give further details.
Andrew Thomas, the Maricopa County attorney, said he believed the evidence against the two was substantial and he promised "to aggressively prosecute these defendants, to seek justice for the victims of these crimes and to fully end this reign of terror."
The police said Hausner and Dieteman had family members in the Phoenix area and might have lived in the city during the spate of killings.
The police in Phoenix are still seeking another serial killer whom they call the Baseline Killer. They say he killed eight people and sexually assaulted 11 others since August last year.