Guilty verdicts against three young New Orleans men in the jarring 2012 murders of 5-year-old Briana Allen and Shawanna Pierce in Central City, among other alleged gang-related crimes, drew a proud response from Mayor Mitch Landrieu late Thursday.

Flanked by District Attorney Leon Cannizzaro and Police Superintendent Michael Harrison, the mayor held a news conference within an hour of the verdicts against Demond “Lil D” Sandifer, 19; Sam “Lil” Newman, 19; and Tyron “T-Man” Harden, 21.

The mayor claimed a key victory for an anti-crime campaign focused on the street gangs blamed for an inordinate share of the city’s persistently high murder rate.

All three men face life prison sentences for the gunfire that killed Briana as she sat on her great-grandmother’s porch and also felled Pierce, a mother of three who took a stray bullet to the head from an AK-47 as she drove a rental car four blocks away.

Landrieu held up a photo of Briana that he said he has kept in his office since the day she was fatally shot in the hip while celebrating a cousin’s birthday on May 29, 2012. The mayor called the murders on Simon Bolivar Avenue a clarion call.

“This shook the city into a position where it could not look away,” he said. “Today we delivered on our promise to the people of the city of New Orleans. We told them we were going to find a new way to identify the individuals of this city who had no regard for themselves or anybody else.”

Tuesday’s verdicts by an Orleans Parish jury closed out the first trial testing a strategy by Cannizzaro’s office to wrap together myriad street crimes in wide-ranging gang racketeering cases involving as many as 20 defendants.

The indictments exploit a once-obscure state law modeled on a federal statute first created to go after the Mafia. Instead of trying individual crimes, prosecutors in the state racketeering cases can present a raft of seemingly disparate crimes and otherwise inadmissible evidence to paint a picture of a gang on the loose.

The targets this time were members and associates of the 110ers, a St. Thomas-area band of teens that Assistant District Attorneys Alex Calenda and Jason Napoli portrayed, over a trial that ran nearly three weeks, as hellbent on bloodshed for the sake of pride and little else.

Most of the other 12 defendants named in the 51-count indictment, which wrapped together 15 murders and several other violent crimes, already had pleaded guilty. Many of them joined a parade of convicts who testified against Sandifer, Newman and Harden in exchange for lower sentences.

The jury of six men and six women deliberated for about five hours before convicting the three men unanimously on a host of charges.

Harden, who prosecutors acknowledged was not a 110er, was the only one spared a connection to gang activity. The jury acquitted him on the main gang racketeering charge — the only count of the 15 that the jury heard against the three men for which it refused to convict.

Sandifer and Newman were convicted on the racketeering count, and all three were found guilty on a weapons conspiracy count.

Newman also was found guilty in the killing of Marlon Smith and the attempted murder of Kevon Robinson on June 14, 2012, two weeks after the Central City gunfire.

Newman, dressed in a V-neck sweater and tie, stood up and sneered as the verdicts were read.

“Bring me to the back,” he said before the judge hushed him.

Family members for the victims and defendants alike burst into tears as the verdicts were read under tight courtroom security shortly before 6 p.m., around the same time of day that four men rolled up on Simon Bolivar Avenue and fired away at a birthday party crowd that included members of the Young Melph Mafia, a rival gang of the 110ers.

Prosecutors told the jury that half-brothers Newman and Sandifer, along with Harden from the 7th Ward, came out firing, with Harden wielding the assault rifle.

The suspected driver, Stanton “Nan Nan” Guillory, is awaiting possible federal charges.

The prosecution stemmed from a lengthy investigation by police and a multiple-agency gang unit that Landrieu’s office set up to cast a wide net over violent crime in the city.

That investigation followed an initial, false arrest and indictment on first-degree murder charges of another 110er, Leo “Nitty” Riles, who testified in the case. Riles was named in the original indictment as the target of an earlier attempted murder by Newman, in 2011.

The jury heard from 58 witnesses, including more than a dozen who stepped to the witness stand in shackles.

Cannizzaro credited a closer partnership with federal law enforcement for helping put together the sprawling prosecution.

“I think we have demonstrated to the city of New Orleans just how far our criminal justice system has come, especially in the last five or six years,” he said. “I think the message is getting out very loud and clear that if you want to go out and wreak havoc on innocent people like Briana Allen and Shawanna Pierce ... we’re going to go after you with every tool in our arsenal.”

Along with the six guilty verdicts against Newman, Sandifer was found guilty on four counts — both conspiracy charges and the murders of Allen and Pierce.

The jury convicted Harden on four counts: the weapons conspiracy charge, the two murders and a charge of illegally carrying a weapon.

Among those testifying against them were Sandifer’s father and Newman’s stepfather, Antonio “Big Rico” Johnson, and Sandifer’s self-proclaimed “Mafia wife,” Ja’On “Sticks” Jones.

Both were named and pleaded guilty to charges from the massive indictment handed up in 2013.

Criminal District Court Judge Tracey Flemings-Davillier set a March 2 sentencing date, although that appears likely to be delayed as attorneys for Sandifer and Newman press for a hearing to avoid sentences of life without the possibility of parole, under a U.S. Supreme Court decision barring such automatic sentences for minors.

Both were underage when Briana and Pierce were killed.

Newman and Sandifer both were already convicted of murder and sentenced to 1 1/2 life terms without the possibility of parole in separate trials last year, for killings pulled from the gang indictment.

Newman was convicted of killing 21-year-old Jonathan “Kruga” Lewis just 12 days after Briana and Pierce were killed. A jury deliberated for less than 40 minutes before convicting Sandifer in August for the 2011 killing of 22-year-old Milton Davis.

Attorneys for the defendants expressed disappointment in the verdicts and said they would prepare to appeal.

Newman’s attorneys, Bradley Phillips and Stavros Panagoulopoulos, said it was a tall order to overcome the boatload of evidence suggesting gang activity: scores of Instagram and Facebook posts of Sandifer and Newman toting guns; hours of incriminating jail phone calls and boastful rap videos; and more than a dozen fellow 110ers and other convicts who tagged the half-brothers as gang members, or who described confessions to the killings.

Jones told the jury that Newman chuckled when describing how he had killed “a whole baby.”

“The racketeering element added so many tools” for prosecutors, Phillips said.

“It was built on stuff they could never, ever have gotten in” by charging the alleged crimes individually, Panagoulopoulos added.

The DA’s strategy has drawn catcalls from defense attorneys, who say the racketeering indictments cast too wide a net, unfairly tagging some defendants as gangsters on the basis of loose ties to other defendants.

To Cannizzaro, it only makes sense for a group like the 110ers.

The jury “did not just hear about a murder case for the last three weeks,” he said. “They got to hear about the armed robberies, discharging of firearms. They got to hear about conspiracy charges that let the jury know the entire and complete and full picture of the activity these members were involved in.”

Follow John Simerman on Twitter, @johnsimerman.