For youths coming of age in the current decade, smartphones are less of a glorified gadget than an extension of the hand. So when Jacob Jensen, 16, witnessed a violent arrest this month involving a friend, he reflexively began filming.

The camera built into his cellphone captured images of a plainclothes deputy punching 17-year-old Brady Becker in the face in the parking garage of the Lakeside Shopping Center.

Jensen, a student at St. Charles Catholic High School in LaPlace, recalled his first thought when Detective Nicholas Breaux took Becker to the ground: “This is really wrong.”

The 37-second clip, uploaded to YouTube last weekend, sparked impassioned debate among thousands of viewers online. While offering just a glimpse of the arrest, the footage could play a key role as the Jefferson Parish Sheriff’s Office investigates whether Breaux, a mixed martial arts fighter, used excessive force in detaining the teenager.

“I knew that I had to send (the video) in somewhere,” Jensen said, “so that justice would be served.”

In the criminal justice system, memories fade and recollections differ, often irreconcilably, tasking judges and jurors with deciphering some version of the truth. But a well-timed cellphone video can be worth a thousand witnesses, depicting a crime as it occurred and, in many cases, generating leads and evidence for law enforcement officials.

“It’s kind of the ultimate truth teller,” said Matt Chester, a former prosecutor in the U.S. Attorney’s Office in New Orleans. “It speaks for itself.”

But the Becker case also highlights the limitations of cellphone footage. The Sheriff’s Office on Friday released a lengthy incident report that alleged Becker had instigated his Feb. 13 arrest by cursing at and punching Breaux amid a drunken fit — accusations that can’t be proved or disproved by Jensen’s brief video.

“That’s the one caveat you always have to deal with,” Chester added. “There are situations where you have a 30-second snippet of something, and in reality, it might have been a five-minute incident. So (the footage) is not always the be-all and end-all.”

The growing ubiquity of smartphones, which make shooting video as simple as tapping a screen, has armed citizens with a new check on law enforcement, particularly in jurisdictions like Jefferson Parish that haven’t implemented body-worn police cameras.

Even a decade ago, before the explosion of the iPhone, a cop’s account of a controversial arrest was far less likely to meet photographic contradiction than today, when more than half of American adults carry smartphones.

“The police should be trained now to assume that they’re always being recorded,” said Howard Friedman, a Boston civil rights attorney who serves on the board of the National Police Accountability Project.

Marjorie Esman, executive director of the American Civil Liberties Union of Louisiana, which has fought to protect citizens’ rights to film the police, said the proliferation of cellphone cameras has made law enforcement officials more answerable for their actions.

“If somebody has a record of what happened, it cuts out a lot of the he-said, she-said,” Esman said. “It does create evidence of police misconduct. It can, of course, exonerate the police.”

Protests erupted around the country last year after a grand jury declined to indict a New York City police officer who placed Eric Garner in a chokehold, leading to his death. A witness filmed the arrest on his cellphone, capturing disturbing footage, viewed millions of times online, in which Garner can be heard gasping, “I can’t breathe.” Federal investigators have opened their own probe into that officer’s actions.

“There’s been a major culture shift in that, 20 years ago, you had to invest in a big, bulky video camera to film things,” said Rafael Goyeneche, president of the Metropolitan Crime Commission, a local watchdog group. “Now it’s smaller than a pack of cigarettes, and you have the ability to shoot in high-definition.”

At a time when everything from dance recitals to cooking forays is memorialized online, police officers are by no means the only subjects of viral cellphone videos. Often, in fact, it’s the perpetrator of a crime who is caught red-handed.

Such was the case in 2013, when Jeremy Hill, then a star running back at LSU, was filmed in the parking lot of a Tigerland bar sucker-punching a 20-year-old man in the head. The grainy video, which showed Hill high-fiving another man after the victim fell to the ground, prompted prosecutors to charge Hill with battery.

In the face of the evidence, Hill had little choice but to plead guilty and serve his probation — a sentence widely second-guessed by armchair jurists who had watched Hill’s misbehavior on their own computer screens. Less than a year later, he was drafted in the second round by the Cincinnati Bengals.

“Without that video, it would have been a much more difficult case, where you have one person’s word against the other and you have alcohol involved,” said East Baton Rouge Parish District Attorney Hillar Moore III.

More recently, New Orleans police had to explain this month why it took 10 minutes for officers to respond to a late-night brawl outside a club on Bourbon Street. A four-minute cellphone video of the fracas, published on Facebook, showed a violent standoff between a group of men and bouncers without any sign of law enforcement intervention.

Police issued an unknown number of summonses after they finally arrived at the scene. But several days later, after the cellphone footage generated local media attention, the authorities arrested four men they said had been planning a “second altercation” at the club, Bourbon Heat.

It’s not clear what impact, if any, the video of Becker’s arrest will have on the JPSO’s internal investigation and the federal civil rights lawsuit Becker plans to file this week. But Becker’s attorney, David Belfield, said he has no doubt the arrest would have garnered far less attention if Jensen had not had the presence of mind to take out his cellphone.

“If I had called you and told you that I got a kid in Reserve that got beat by (a Jefferson Parish deputy), I wouldn’t have all these cameras in my face,” Belfield said during a news conference Thursday.

The taping of the arrest, he said, “was probably the best thing that happened to Brady in this case.”

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