At a recent after-school session in Anne Arundel County, Detective Mark Finley showed a group of teenagers and their parents how violent gangs recruit in their community, playing a rap video by members of the notorious Bloods gang from California.

When he asked the teenagers how many had seen the video on YouTube and noticed Bloods’ graffiti scrawled at their schools, almost all of them raised their hands.

No surprise there.

Finley and his fellow officers started noticing the gangs on sites like YouTube, Twitter, MySpace and Facebook about five years ago, he said. Social media have given them the ability to communicate swiftly and boldly and recruit members across geographic boundaries, making turf all but meaningless.

The officers are now so used to watching gangs do business online that they have created their own fake social media profiles to go undercover.

To make this point, Finley pointed to another Anne Arundel cop, white and middle-aged. “See this guy right here?” the detective asked. “On Twitter, he’s an 18-year-old African American Bloods female.”

A 2012 statewide assessment of gangs by the Maryland Coordination and Analysis Center, an organization that comprises local, state and federal law enforcement agencies, found an overall increase in gang activity over the past decade, with many gangs being pushed into Anne Arundel and other suburbs in Montgomery, Prince George’s and Charles counties as a result of gentrification in the District and Baltimore.

Wearing an oversize T-shirt and a skull cap over his shaven head, Finley looks and talks more like a gang member than a cop, a product of his upbringing in a rough area of Los Angeles, so his message resonated at this after-school session for at-risk teens in Annapolis.

He is a member of the Mid-Atlantic Regional Gang Investigators Network, which brings together cops who police gangs across the Washington region. He travels widely and speaks to police, parents and kids about the power that gangs command when they use social media.

At this session in Anne Arundel, Finley told of local teens getting beaten up by members of national gangs such as the Crips whom they met on MySpace. He described gangs in neighboring Prince George’s County using Twitter to find out where school-skippers were hanging out and then showing up to recruit them. He explained how smaller, local gangs, or “crews,” plot their crimes online, where law enforcement is watching them.

“The new era gangster realizes it’s not about territory, per say; it’s about membership,” Finley said. “They don’t care what gender you are, they don’t care how old you are, what color you are, what anything you are. The only thing they care about is recruiting you into their gang. That’s it. That’s why we have Crip sets now that are popping up in Anne Arundel County that came out of Southwest Baltimore.”

The teens know local crews by name and by neighborhood. Unlike national gangs, who are moving away from controlling territory, crews in Annapolis still keep to their communities. But they are not immune to the effects of social media and have begun to threaten each other and switch affiliations online, according to prosecutors.

Ten years ago, when Finley left his home in L.A. to move to Anne Arundel County, he thought he knew gangs and that his days of busting them were over. Anne Arundel’s most urban area is Maryland’s historic capital of Annapolis.

But in 2005, he began to notice the same graffiti he saw back in L.A. Scrawled under bridges, in parks and on school property across the county, the spray paint bore the unmistakable signs of MS-13, a violent Salvadoran gang that began in Los Angeles and has now gone international, and a subgroup of the Bloods gang called the Tree Top Piru.

In L.A., Finley could identify which gang someone belonged to by their neighborhood. In Anne Arundel, it appeared, gang recruitment was a different game.

He asked himself how gangs could spread not only across state and national borders, but how they could end up in a place like Annapolis, where there was no history of gang violence.

On Sept. 4, 2008, he got his answer: Social media.

Finley was called to the scene of an assault in Millersville, about 15 minutes north of Annapolis.

According to Anne Arundel County Circuit Court records, two brothers, 16 and 19, were badly beaten in their front yard by four or five men. Jeffrey Holbrook, then 20, a leader of a Crips subgroup or “set,” the Rollin’ 60s from Glen Burnie, near Baltimore, came to their home to initiate one brother into his gang, the records say.

Though Holbrook was on house arrest for a previous crime, he could run gang operations over the Internet and use MySpace to contact the younger of the brothers, according to the court records, which provide the following account:

Through MySpace messages, the younger brother told Holbrook that he wanted to join the Rollin’ 60s. Holbrook gave him two options for initiation: He could commit two crimes or be “beat in.” The teenager chose the latter, and Holbrook showed up to carry out his initiation the day after he was released from house arrest.

The teen quickly backtracked and told Holbrook he no longer wanted to join, but it was too late. As a gang member held him down and repeatedly punched him in the head and body, his older brother came to his defense. But then Holbrook called out “hootie hoot,” the Crips’ call, and three other men in blue bandannas (the Crips’ signature color) emerged from the woods and held the older brother to the ground while he was choked, kicked and gouged. The brothers, powerless, suffered until their Rottweiler came running and the attackers disappeared in a getaway car.

Holbrook and another gang member were convicted in June 2009 of first-degree assault, second-degree assault and reckless endangerment. Holbrook is serving four years in prison.

“That was the first [social media] case that we had,” Finley said. “At that time nobody was paying attention to MySpace. Now you see it all the time.”

Local prosecutors have also learned to search social media sites for evidence. When Assistant State’s Attorney Michael Dunty prosecuted Holbrook, he used Holbrook’s MySpace page as evidence of his gang affiliation.

Simply being a member of a gang — and showing that allegiance online — is not illegal in Maryland. Under state law, a person must commit a crime and also be found guilty of acting in coordination with a gang in order to be sentenced for gang affiliation.

Federal law requires only gang affiliation — not a committed crime — for a conviction. For that reason, local jurisdictions in Maryland often turn to federal prosecutors for help in combating major gang leaders, said Rod Rosenstein, the U.S. attorney for Maryland.

At a recent meeting in Prince George’s County for young men who are at risk of joining gangs, six former gang members explained how the Internet has become “the new street corner.”

Those in attendance, all young African American men, are involved with Higher Hopes to the Outcomes, a local nonprofit group that serves as an after-school center for kids from troubled neighborhoods. The group produces rap and hip-hop with a positive message and puts the recordings up on SoundCloud and YouTube, next to those produced by gang members.

The former gang members explained that YouTube and Facebook are places where the Crips and Bloods can demonstrate dominance and earn popularity through “likes” and views.

“You either gonna be a man or a mouse. If you a mouse, it’s like game over,” said Jovan Davis, 27, of Washington, D.C. who has seen gang members make threats on YouTube.

“That’s the thing,” added Stanley Ramey, 18, “sometimes you got to do bad to get respected out here.”

Bruce Parnell, founder of Higher Hopes to the Outcomes, said he is starting to see the early signs — graffiti and hand signs — of gang affiliation and turf wars in Anne Arundel County, which is why he is opening two youth outreach centers in Annapolis.

The former gang members at Higher Hopes say the loose structure of Anne Arundel County gangs and crews allows for more serious gangs from Prince George’s County, the District and Baltimore to come in and assert dominance. Without one group controlling the drug trade or the block, the county is perfectly situated to be taken over by a stronger gang from the outside.

“They don’t really know about that lifestyle in Anne Arundel,” said Michael Ramey, a young father who grew up in Prince George’s County neighborhoods with gangs. The outside, nationally affiliated gangs have “an advantage over them.”

Ramey said he knows the lessons Finley is teaching in Annapolis: Be careful whom you talk to online, never rely on a gang for loyalty and protection — and do not attempt to earn respect through violence.