Phoenix ? With two serial killers on the loose in Phoenix, Marnie Reiher knows she should stay off her front porch. She shouldn’t answer the door. And if the cats sneak out at night, she should leave them on their own till morning.

“But then I’d be giving in,” said Reiher, 36, who refuses to hunker down even though she lives a few blocks from where one of the killers struck. “I’d be imprisoning myself.”

It is a common feeling in this city of 1.5 million, where gunmen have randomly shot dozens of people since May 2005, killing 13. While many still shutter themselves inside their homes, a growing number have decided to fight back.

They are patrolling their neighborhoods at night, cell phones and emergency whistles in hand. Some have started new block watch groups, while others have donned the red berets and white T-shirts of the Guardian Angels, who are starting a chapter here.

At community meetings, women remind each other of the safety advice they heard while growing up: Squeeze a car key between your fingers and you have a knife. Wear your purse in the front so someone can’t strangle you with the strap. Keep your head up. Make eye contact. Kick him in the groin.

Police have no suspects in either case.

They believe the attacks started more than a year ago, beginning with a gunman who fires from a car and has been dubbed the Serial Shooter. That assailant is thought to have killed five people, wounded 17 and targeted horses and dogs, too.

Another predator known as the Baseline Killer, so-called because some killings took place near Baseline Road, is thought to be responsible for eight slayings and 11 sexual assaults since August. In all but one case, his victims were women. He attacks at close range.

The Baseline Killer tends to strike in the late afternoon and evening, while the Serial Shooter usually opens fire during the overnight hours.