SIXTEEN DAYS before Frank Zito is charged with murder, Donna Dukes hears a knock at the door of her Centreville apartment. She's expecting company, so she doesn't check the peephole before opening the door.

It's Frank Zito again - only Dukes, a 49-year-old housekeeper, doesn' t know it's him. To her he's the stranger in the knit cap who came to her place an hour earlier, looking for one of her neighbors. He made her uncomfortable the first time, and now that he's back, Dukes is uneasy.

"What's your name?" he asks.

There's something unsettling about the man. He's 6 feet tall and unshaven, with heavy eyelids and large ears that don't lie flat. But it's not really his appearance that puts Dukes on guard: It's the way he stares at her. She doesn't want him to know her name, but she worries that she'll set him off if she doesn't tell him.

"Donna," she answers.

The man pulls the cap off his head, as if trying to be polite, leaving his hair in disarray. He takes a step forward. She makes up a story - her daughter's on the phone, she has to go - and shuts the door.

She watches through the peephole until he walks away.

A few nights later, Dukes comes home around 10:30 and finds a tiny piece of paper covered with writing stuck in her door.

"Dear Donna, I was so impressed to meet you. I'd like you to consider me going out with you, or taking you out or maybe your girlfriend too, both of you. I think I like you a lot." There is a phone number and directions to the man's trailer. "Just stop by ... Please!"

The note is signed "Frank Zito."

It takes a minute to register, but when she realizes Zito was the man at her door, Dukes feels sick - and violated. What right does he have to intrude on her life this way? And how can she prevent it from happening again?

Dukes is hardly the first person in town to ask those questions. Zito, a 41-year-old Centreville resident with a severe mental illness and a history of resisting treatment, has rattled countless people - many of them women - with his actions and words. And tonight won't be the first time that a citizen upset by Zito's behavior will turn to the Centreville Police Department for protection and help.

Two young patrolmen respond to Dukes' call, including Officer Michael Nickerson, the department's newest hire, on the job just over a week. According to Dukes, the officers suggest she not sleep in her apartment alone that night. Told she has no place else to go, the policemen say they'll patrol her parking lot. They also advise her to seek a court order to keep Zito away from her. Finally, at 1 in the morning, according to Nickerson's report, the two officers "located Frank Zito on Chesterfield Ave. and advised him not to have any contact with Dukes."

But Dukes is still frightened. The next day, she talks with Police Chief Benjamin Cohey, who lets her know that despite Zito's repeated and unwanted displays of attention to women, he has never done anything to hurt those he pursues.

Twelve days before Zito is charged with shooting and killing two people, Dukes finds the chief's words reassuring.

YOU DON'T HAVE TO GO FAR in Centreville to find someone who knew Frank Zito. He was as much a fixture in this town of 2,200 as the bronze statue of Queen Anne on the lawn of the courthouse square, and - it seemed for years - just as immovable. No matter how many times he went away for psychiatric treatment, he inevitably ended up back home, in the trailer park run by his mother in the neighborhood near the Corsica River known as the Wharf.

Though Zito's off-putting, disruptive behavior wasn't an everyday occurrence, it had long drawn the attention of local law enforcement officials. He'd been convicted on misdemeanor charges several times. But mental illness isn't a crime, and misdemeanor charges aren't murder. Before the Feb. 13 shootings, some people would no more have predicted the gunfire at Zito's trailer door than they would have expected a pothole in town to open up and swallow a car, or a leaky pipe to unleash a tidal wave. "Crazy Frank" was a nuisance they had learned to live with, steering around his occasional outbursts, plugging the leaks when he seeped into their personal space.

But others - particularly in the neighborhood surrounding his trailer - had feared for years that Zito was a disaster waiting to happen. Unnerved by his unpredictable behavior and exasperated by their inability to control it, they had come to wonder if it would take a tragedy for something to change.

Francis M. Zito's future is now a question for the courts to decide, when and if he's found competent and the case goes to trial. But Zito's past - and how it might have been different - is something that people in Centreville and elsewhere have struggled to come to terms with for months now. Who could accept the fact that nobody had defused a bomb that had been ticking away, sometimes loudly, for the better part of a decade? Who hasn't wondered what sort of intervention might have prevented the loss of two young lives?