By the time a group of teenagers poured into the streets on their way to the funeral of an alleged Brooklyn gang leader known as “Freshh,” a social networking Web site, MySpace.com, had already been flooded with tributes to him.

Dozens of personal MySpace pages displayed pictures of 18-year-old Donnell McFarland — who police say was shot by rival gang members last month — adorned with digitally added angels’ wings and Web page titles that incorporated the phrases “R.I.P. Freshh” and “Miss U.”

Some of the keenest viewers of those pages may have been law enforcement officials. Trolling social networking sites such as MySpace, Facebook, and Sconex has become standard procedure recently for police and prosecutors, who have found the Internet to be a new and increasingly indispensable tool for investigating local youth gangs.

When police arrested about 30 of McFarland’s friends en route to the funeral on charges of illegal assembly, they said they had been investigating the group McFarland allegedly led, the Pretty Boy Family, for some time. Surfing the network of MySpace sites diligently maintained by Pretty Boy Family members was likely a part of their process.

Following the lead of employers, parents, schools, and reporters, law enforcement agencies have discovered the Internet’s treasure trove of information about the private activities of young people who post the intimate and occasionally incriminating details of their lives for all to see.

“It’s one of those tools that is new to investigators because of technology,” a spokesman for the New York Police Department, Assistant Chief Michael Collins, said. He said the police use it “in certain cases,” and added that detectives were unwilling to disclose anything more specific about their investigative techniques.

Prosecutors have also begun to frequent the sites, using them to gather evidence that can be used in court. “Now it’s a fairly routine,” a spokeswoman for the Manhattan district attorney’s office, Barbara Thompson, said.

In the case of youth gangs, the sites are shedding light on what has generally been a secretive world. Posts include information about everything from the symbols and slang used by gang members to hints about their involvement in drugs and violence.

A young victim alerted a lawyer in the office of the Queens district attorney, Richard Brown, to the crime-fighting potential of the sites. An assistant district attorney, Shlomit Metz, had been prosecuting the hard-to-crack case of a young man attacked by a group of strangers at an Astoria hot dog stand.

“I saw it as a gang jump, but I couldn’t prove it,” Ms. Metz said. After months of digging, Ms. Metz said she had turned up nothing — until the victim suggested she look up the defendant’s MySpace page. There she found photographs of the defendant holding a gun and brass knuckles, and wearing the gang colors of the Zulu Nation. She also found connections to dozens of other Zulu Nation members across the country among his network of friends.

Ms. Metz presented printouts of the page at the defendant’s sentencing in March 2006. After losing an appeal, he was sent to prison for seven years.

Now Ms. Metz says she and the other attorneys regularly surf MySpace for evidence. “Whenever I have a young perp, I do as many searches as I can,” she said. “I try to utilize it to show: Here’s proof that they’re lying, here’s proof of their gang affiliation.”

The Pretty Boy Family’s “official” MySpace page is plastered with photos of the group flashing hand signs and wearing matching red clothing.

But gang experts warn that information on social networking Web sites can sometimes be misleading, with teenagers tending to pose as tougher or more street savvy than they are.

The co-director a Bushwick community organization, Make the Road by Walking, Oona Chatterjee, works with Brooklyn teenagers who post on social networking sites, including several of McFarland’s friends.

“There’s a lot of posturing that happens because life in our neighborhoods is difficult,” she said. “A lot of times they’re stretching the truth.”

McFarland’s friends and family have said police are wrong to call the Pretty Boy Family a gang, saying it is just a group of friends. Many of the recent MySpace postings by members of the group are affectionate ramblings about McFarland.

Only one anonymous entry on the official Pretty Boy Family page, posted the day of McFarland’s death, appears to invoke violence. It warns, “all haters get flat line early.”