The group looked for easy pickings in crowded places, scanning Chinatown, Dupont Circle and the U Street corridor for people talking distractedly on cellphones or convenience stores that could be descended on “flash mob” style. Then, authorities said, they’d rob or steal, assaulting victims and intimidating shop workers.

Sometimes, they’d even attack sleeping homeless people, according to authorities.

On Thursday, authorities said, eight District men and one woman believed connected to the attacks were indicted on 30 criminal counts in the series of robberies, thefts and violent assaults throughout 2012.

“This was a very disturbing case,” D.C. Police Chief Cathy L. Lanier said at a news conference. Police “still have a lot of work to do” and further arrests, perhaps of individuals as young as 15, may be forthcoming, she added.

The suspects were part of a gang whose members called themselves the Show Out crew, according to authorities. Many were also members of another crew made up primarily of gay and bisexual teenagers, Lanier said. She did not name that gang.

They targeted Foggy Bottom, Bloomingdale and some Montgomery County neighborhoods, officials said, using public transportation and sometimes hitting two or three businesses in a day.

“These defendants are accused of robbing, assaulting and terrorizing citizens in some of the most vibrant parts of our city,” U.S. Attorney Ronald C. Machen Jr. said in a statement.

The Chinatown area has exploded into one of the city’s most active night-life districts, and police have admitted that they initially failed to recognize the area’s popularity, challenging their crime-fighting efforts there.

Jo-Ann Neuhaus, director of the Penn Quarter Neighborhood Association, said there has been a vast improvement in police response since large groups of youths became an issue for residents, businesses and visitors three years ago.

But large crowds still gather on Seventh Street, particularly outside Verizon Center, attracting continued police attention.

“It’s scary when that kind of behavior becomes associated with a large number of kids,” Neuhaus said.

Named in the indictment were Bernard Trowell, 19, whom police called the group’s “self-styled president”; Deandre M. Williams, 18; Quayshawn L. Leggett, 20; James D. Matheny, 18; Travis L. Morris, 20; Brandon J. Steele, 18; Wayne E. Wheeler, 20; Ricardo J. Williams, 20; and Angel Collier, 18.

The defendants live in various parts of the District and, according to Lanier, met and organized using social media, including Facebook and YouTube.

In the 23-page indictment, prosecutors said that on June 5, members of the “crew” targeted a Hispanic man about 3:45 a.m. near Second and K streets NW. They allegedly used racial epithets and, as the victim tried to give them his wallet, said, “We don't want your money” and continued the assault.

That attack happened about 20 minutes after another man was allegedly beaten and robbed by the group in the 600 block of H Street NW. In that attack, according to the indictment, the attackers tried to hide the victim’s unconscious body underneath a bus depot.

Along the U Street corridor, the head of the neighborhood association, Brian Card, said police have discussed feuds between nearby gangs, including one on Sept. 11 that led to 20 shots being fired outside an elementary school as students streamed out of the building in mid-afternoon.

But Card said he didn’t know about an organized robbery gang before learning of the arrests Thursday.

“Personally, to have a sense of why things are happening is good,” he said.

Six of the defendants were arraigned in D.C. Superior Court on Thursday. The rest are expected to make their first court appearances soon, prosecutors said.