Lopera called the arm-around-the-neck maneuver a "rear naked choke," Clark County Undersheriff Kevin McMahill said. It is not authorized by Las Vegas police.
McMahill said the hold differs from a department-taught technique called "lateral vascular neck restraint" or a carotid artery hold that proponents say does not impede breathing but instead restricts blood flow to the brain and causes loss of consciousness.
That method, which McMahill said can render a person unconscious within 10 seconds, is banned for use by police in many other cities, including New York, where the death of Eric Garner on a Staten Island sidewalk in July 2014 sparked a national debate about police brutality.
Records show Las Vegas police officers used the neck restraint 632 times over a 10-year span, including 51 times last year. That's about once a week.
In Los Angeles, by comparison, the carotid hold is considered lethal force only to be used in life-and-death situations. Police there reported using it seven times in the last five years, including twice in 2016.
It could take several weeks to receive full test results and determine what caused Brown's death early Sunday, the Clark County coroner said.
Brown appeared to be sweating and agitated when he approached Lopera and a partner in a casino coffee shop. He said someone was chasing him and ran outside to a driveway, McMahill said.
The family attorney said Brown usually took prescription medication for depression, but he did not know if Brown had taken it Saturday night. Brown's mother said Brown was home when she went to bed. She noticed him missing Sunday morning.
In the casino driveway at 1 a.m., Lopera thought Brown was trying to carjack a pickup truck with two people inside, McMahill said. The officer fired the stun gun in a series of staccato bursts.
The pickup driver later said he did not think Brown posed a threat, McMahill said.
Video shows Brown stiffen and fall backward onto the pavement with his arms raised. Lopera shouts at him to get on his stomach. Brown rolls to his side as the officer fires another jolt from the stun gun.
"I will!" Brown cries out, "Please! Please!"
Lopera, who is white, and Brown, who is black, then wrestle as the officer attempts to handcuff him. Lopera punches Brown's head and neck from behind as Venetian security guards join the effort and other police officers arrive.
Sheriff Joe Lombardo told The Associated Press there was no indication that race played a role in the struggle.
But Lagomarsino said race could not be ignored.