A 5-year-old girl lay dying in her father’s arms, her femoral artery torn through by a bullet from an AK-47 as she sat on a porch celebrating a young cousin’s birthday.

Several blocks away, a 33-year-old mother of three who had been driving through the neighborhood was already dead in the driver’s seat of a rental car from a stray bullet to her face.

The high-powered gunfire that claimed the lives of little Briana Allen and Shawanna Pierce in violence-plagued Central City on May 29, 2012, sent shivers through the community and thrust police into action.

They promptly tagged 18-year-old Leo “Nitty” Riles as one of the three gunmen, based on a witness who fingered him in a photo lineup as the shooter with a mohawk haircut.

Riles surrendered the next day, and a grand jury indicted him on two counts of first-degree murder — charges that authorities later concluded were bogus.

Yet Riles’ false arrest, a state prosecutor told a jury Wednesday, marked only the start of a massive, multiple-agency investigation that bored into the rampant violence of a crew of young gang members with roots in the River Garden Apartments neighborhood and tentacles into the 7th Ward.

That investigation, taken up by a Multi-Agency Gang Unit launched by Mayor Mitch Landrieu’s office to root out the bloodiest of the city’s violent street criminals, led to a massive state grand jury indictment in 2013 that accused 15 alleged members of the 110ers street gang and associates in a racketeering conspiracy that drew together 15 murders and numerous other killing attempts over several years.

Along with the slayings of Briana Allen and Shawanna Pierce on that May afternoon three years ago, they included an attempt, 15 months earlier, on Riles, himself an alleged 110er, according to prosecutors.

His alleged assailant, Sam “Lil” Newman, and two others sat quietly in an Orleans Parish courtroom Wednesday as prosecutors launched into a trial that is expected to crack a window on one of the city’s most notorious street clans — at least as prosecutors describe it, backed by a parade of former co-defendants and other convicts expected to testify after securing plea deals.

The gang racketeering case against Newman, 19; Demond “Lil D” Sandifer, 19; and Tyron Harden, 21, is the first to go to trial among a half-dozen such cases that District Attorney Leon Cannizzaro’s office has pursued since late 2010.

That’s when the DA, aiming to cast a wider net over street violence in the city, turned to an obscure state statute that mimics the federal RICO statute first created to go after the Mafia.

So far, those cases have netted numerous plea deals, even as defense attorneys grouse that the indictments reach too far by coloring individual acts and defendants as parts of grand schemes.

Along with expected testimony from more than dozen convicts in a trial expected to run a few weeks, prosecutors are expected to unload a massive trove of evidence on the jury in a case that attempts to weave seemingly disparate killings and other violent acts, along with armed robberies and gun counts, into a tapestry of brutal gang-driven activity.

The jury will see scores of Facebook and Instagram posts, view homespun rap music videos and hear numerous taped jail phone calls in which prosecutors say the accused boasted of their violent exploits with laughing abandon. Much of the evidence, Assistant District Attorney Alex Calenda said, will focus on cellphone calls and locations that tell a clear story of a criminal gang at work.

During his opening argument Wednesday, Calenda described a group that saw violence as a means for preserving its power and street prestige, or perhaps as its own reward.

Although the 51-count indictment credits Newman and Sandifer with several murders — five for Sandifer and four for Newman — Calenda focused mostly on the high-profile killings of Allen and Pierce. The victims “didn’t realize, as many others didn’t, that there are nightmares who walk this city in a culture of violence that pulses through their veins, that consumes their being, that drives their very actions and their very thoughts,” the prosecutor said.

Most of the other defendants in the case have pleaded guilty. Among the few remaining to be tried was the fourth man who allegedly participated in the attack that killed Allen and Pierce. Stanton “Nan Nan” Guillory is expected to be charged in federal court, attorneys said.

Newman and Sandifer seem to have little to lose in going to trial. Both were convicted last year of murder and sentenced to 1 1/2 life terms without the possibility of parole for killings that were culled from the gang indictment.

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

Sandifer’s father, Antonio “Big Rico” Johnson, testified in that trial, identifying Sandifer and another indicted son, Rico “Max” Newman, as 110ers. Johnson, who received an eight-year prison term after pleading guilty in the racketeering case, will testify again in this trial, Calenda said.

The 110ers were made up of three groups — the St. Mary Mafia, St. Thomas Young’ns and Skull Squad Mafia — with their stomping grounds in the 10th and 11th wards. Some young members espoused their own sub-brand, “Team Murder,” Calenda said.

Among their rivals were the “Get Money Boyz” from the 12th Ward and the Young Melph Mafia from the 3rd Ward, according to the indictment.

Members of the 110ers group associated with the Young Mafia Fellaz from the 7th Ward. That’s how prosecutors say Harden, who was not a 110er, got hooked into the group that went looking for rival blood on May 29, 2012.

Calenda said many of the associates first bonded together at Algiers Tech, where Sandifer had gone to school.

Harden’s attorney, Rick Schroeder, told the jury that hard evidence against his client is scant. No witnesses have identified him in the shooting that claimed Allen and Pierce, and he’s named in just a few of the 89 “overt” acts that buttress the main, racketeering conspiracy count in the indictment.

Along with the murders of Allen and Pierce, those acts include fleeing from a police officer while armed, possession of a handgun and giving a false address. None of the guns found on Harden was linked to the crimes alleged in the indictment, Schroeder said.

“They just want to tar him with the fact he doesn’t live a perfect life, didn’t live a perfect life,” the attorney said.

Prosecutors went overboard “to give the city of New Orleans the alleged four people who killed that little girl and that passer-by on May 29, 2012,” he said.

“What will be missing at the end of this trial is any reliable testimonial evidence to support the story that will be given by the gang of liars they will be putting on the stand.”

Among those that Calenda said will testify is Burnell “Baldy” Allen, Briana’s father.

The photographed image of Allen’s sweat-covered face as he cradled his daughter’s dying body on the porch in the 1200 block of Simon Bolivar Avenue became a symbol of despair in the face of the city’s senseless street violence.

A federal jury in November convicted Allen, 35, and two cousins on a drug conspiracy count for what prosecutors described as a family-run drug ring centered at the former Melpomene public housing complex.

What’s lacking in the racketeering case is a motive for targeting Burnell Allen, said Bradley Phillips, one of Sam Newman’s attorneys. He described the state’s case as a “smear campaign.”

“You won’t hear about any reason Sam Newman had to go and shoot at that birthday party,” Phillips said.

Follow John Simerman on Twitter, @johnsimerman.