In some high-profile cases, the flash mobs have been used by gangs of youths to carry out the group beatings of strangers. On Sunday, a flash mob was blamed for a riot that broke out in Baton Rouge, La., where 200 teenagers engaged in a fight, causing the mall to be evacuated.
Law enforcement is also increasingly perusing social media sites to learn more about gang activity and get a better sense of when retaliation among certain groups will strike. For example, last year, police departments in Chicago, New York, and Philadelphia announced units to investigate social media behavior among gang factions, which often use mobile technology to plan, and later brag about, violent acts related to turf battles.
In Chicago, the strategy was used to investigate Keith Cozart, a rap star known as Chief Keef, who bragged on Twitter after a rival was gunned down in September. Mr. Cozart was also known for YouTube clips in which he mocked the slain victim.
Another local rapper named Lil Reese, whose real name is Tavares Taylor, came under scrutiny in October following the release of an online video to multiple hip-hop sites that show him severely beating an unidentified woman at a party. He was not charged because the woman could not be identified.