The observant citizen noticed the black truck parking in about the same place several days a week.

It fit the description, and the license plate matched, too.

And the burly man getting in and out of the Ford F150 looked like the suspect police sought: 6 feet tall, 230 pounds, short blond hair and hairy arms.

But the vigilant resident didn't call police and instead logged onto CitizenObserver.com. The Web site had first warned people Jan. 16, via e-mail and text message, to watch for 23-year-old Christopher Malinak.

And it allowed one watchful person to e-mail an anonymous tip to Pittsburgh police, pinpointing the location of Mr. Malinak, who was charged with trying to lure two children into his truck on Jan. 15 in Brighton Heights. The children, ages 9 and 11, had been on their way home from school.

Thanks in part to that tip, U.S. marshals arrested Mr. Malinak, of Whitehall, Jan. 18 outside Loew's Theater in West Homestead. He was apprehended fewer than 48 hours after police sent his mug shot and a detailed physical description directly to the inboxes of the more than 3,500 people who'd signed up for the program, an increasingly popular community policing tool.

Launched about a year ago as a pilot program sponsored by community groups and used only by police on the North Side, Citizen Observer expanded to a citywide investigation and community policing tool late last year.

Police using the system can send "alerts" quickly to residents throughout the city or to target groups, such as neighborhood organizations or businesses.

The program is a welcome advancement because police constantly ask the community to be their eyes and ears, said Cmdr. Catherine McNeilly, who supervises the North Side station, which started the pilot program in February 2007. And Citizen Observer helps police quickly communicate with residents interested in helping. It tends to be a highly attentive audience because everyone receiving the information asked for it by signing up.

"It's just one more tool for pushing information out to the community when we need to," Cmdr. McNeilly said. Police can include a photograph of a suspect along with a written description, or even a surveillance video in the case of a robbery suspect.

The department will pay $55,000 over the next two years to use the program citywide, police bureau officials said. Officer Forrest Hodges, who administers the program for Zone 1 on the North Side, said the program would be worth that much if it helped in just a handful of investigations because of the way it has improved community outreach. Officer Hodges used to spend hours, night after night, attending community and watch group meetings to distribute information.

Now he can do it in seconds, not only to warn residents or to obtain information about dangerous suspects. Police at the Zone 3 South Side station and Zone 4 Squirrel Hill station used the system to alert residents of an increase in thefts of car windshield devices, such as GPS navigators or satellite radio receivers.

Pittsburgh has joined a technological tidal wave of more than 300 departments signed up for Citizen Observer, said Terry Halsch, president of the St. Paul, Minn.-based company.

Law enforcement experts have lauded the program as an example of police agencies becoming attuned to the benefits of sharing information with the communities they protect.

"If you think about solving crimes, one of the most important elements of being able to solve that crime are witnesses and information from the community," said Jeremy M. Wilson, associate director of the Center on Quality Policing at the RAND Corp. A system that improves communication between residents and police can help solve crimes, he said.

The program serves as an investigation and community outreach tool for departments as large as Boston's, with 2,500 officers, to small towns with fewer than 15 full-time, Mr. Halsch said.

As with seemingly everything it touches, the Internet has affected community policing.

The emergency management agencies of some cities and counties are experimenting with rapid text-message warning systems, particularly in natural-disaster-prone areas, such as the Florida coast, said Robert D. McCrie, a professor at the John Jay College of Criminal Justice who has studied community policing strategies. After the Virginia Tech massacre last year, colleges and universities across the country implemented similar systems.

"It represents a new tool that can be helpful in involving the public in law enforcement," Dr. McCrie said. "I see more and more of that inevitably, using technology and rapid communication for community policing."

Law enforcement experts agreed that departments should be fastidious and selective about what information gets out, however.

Sending too many alerts might desensitize recipients, who might eventually be inclined to gloss over an alert of actual importance, treating it as a just another piece of spam.

"If you're getting 20 e-mails a day, it might become difficult to pay attention to any of them," Dr. Wilson said. "If it gets to be chatter in the background, it might be something people won't look at."

Police should also be careful to provide the community with only detailed descriptions of suspects that could realistically aid in making a positive identification, not to send out a vague accounting of age, race and gender.

"If the characteristics of a suspect are too general, an innocent person is arrested," Dr. McCrie said.

Cmdr. McNeilly said that detailed policies and procedures for how to use the program are not yet in place.

"Once we're more skilled in using the program, we'll be able to respond better," she said.