Police arrested more than two dozen suspected Portland gang members and associates after a federal grand jury indicted some of the city's most violent elements, authorities said Tuesday.

PORTLAND — Police arrested more than two dozen suspected Portland gang members and associates after a federal grand jury indicted some of the city's most violent elements, authorities said Tuesday.

Federal and local officers fanned out in the past week to arrest 13 people on charges that included sex trafficking and distribution of cocaine. The majority of the charges involved felons in possession of weapons.

Some of the 31 suspects who were arrested are facing state charges.

Oregon U.S. Attorney Amanda Marshall said it was necessary to bring federal charges as the city grapples with gang violence.

"I was really struck by the enormity of the problem," Marshall said.

So far in 2011, 402 gang members have been arrested on charges that carry mandatory minimum sentences under Oregon's Measure 11 law, which specifies the most serious violent and sexual assaults, police said.

The investigation yielded the names of 68 people with alleged gang ties.

The city's most violent gang is now the Rolling 60s Crips, an offshoot of a street gang that originated in Los Angeles, Police Chief Mike Reese said.

"This is a generational gang," Reese said. "It has had historical ties to this area."

Reese said the police force would now focus on "hot gangs" and "hot families" identified as being involved in ongoing criminal activity.

Assistant Portland Police Chief Eric Hendricks, who was once a lieutenant on the gang beat, said gangs generally emerged in the city in the late 1980s and had links to Southern California gangs.

The problems peaked in the late 1990s, when Hendricks said police responded to more than 1,000 incidents of aggravated assault involving handguns each year. That number has now dropped to the low hundreds each year.

This year saw the death of a 13-year-old boy who claimed an affiliation with a Mexican-American street gang called the Surenos.

Portland police have tried to stem the violence by dispatching response teams to incidents that might involve gangs — mostly shootings, stabbings and serious assaults. Gang-related deaths have remained steady in Portland since 2000, ranging from two a year to a high of seven in 2003, 2004 and this year.

In October, the fatal shooting of 25-year-old Deandre Clark set off a series of retaliatory shootings, culminating in the arrest of four men who police said were driving with a loaded .357 magnum in plain sight.

In September, six teenagers between the ages of 13 and 17 suffered gunshot wounds after violence broke out in the northeast part of the city. Investigators learned that two men had fired into a crowd of people and fled.

At the time, Hendricks referred to the attack as an ambush.