The sky was dark, but extra lights along Del Playa Drive in Isla Vista illuminated the street enough so a group of police officers could see a fistfight between a handful of young people on April 5.

Wearing his dark navy uniform, UC Santa Barbara Police Officer Antonio “Tony” Magaña approached the group in the 6700 block of Del Playa about 9:30 p.m.

Two Santa Barbara County sheriff’s deputies and a civilian walk-along came, too, all staffed in anticipation of Deltopia, an annual unsanctioned, alcohol-fueled event in the student-dominated community adjacent to UCSB campus.

The young people immediately bolted, running in opposite directions down Del Playa, still crowded with pockets of people during the weekend event that drew nearly 25,000.

Magaña, a nine-year veteran of law enforcement, zeroed in on a slim male wearing a white T-shirt and pinkish shorts, and sprinted after him.

So began a foot chase that led to his alleged assault by Desmond Edwards, a 17-year-old Los Angeles resident accused of swinging a backpack full of liquor bottles into Magaña’s face — causing an injury that required 20 sutures to close the wound.

The incident unfolded in seconds, but Magaña recounted the event over several hours Wednesday during the first day of Edwards’ preliminary hearing in Santa Barbara County Superior Court.

Edwards, who is being charged as an adult, faces felony charges of assaulting a police officer, mayhem, resisting arrest and causing great bodily injury. He pleaded not guilty April 14.

The Sheriff’s Department has called the alleged assault one of the triggers for the violence that broke out at Deltopia after officers swiftly responded.

On Wednesday, Edwards wore glasses, a green button-up shirt and a plaid tie, along with ankle handcuffs and a set on his wrists that Judge Brian Hill allowed to be removed. Media weren’t permitted to take Edwards’ picture because he’s a minor.

Senior Deputy District Attorney Mary Barron walked Magaña through questions, establishing that he worked as a Guadalupe Police officer before joining UCSB Police in late 2009.

Magaña said he and other officers identified themselves when approaching the subjects and told them to stop. Magaña and a deputy chased Edwards and a second runner, who quickly sprinted out of sight on westbound Del Playa.

When the other deputy collided with a female pedestrian — both falling to the ground — Magaña kept after Edwards.

“It appeared he was carrying something, and I believe it was possibly the reason he wasn’t running as fast,” Magaña said, describing a backpack.

Edwards ran on the sidewalk and twice changed directions while Magaña followed on the street, he said. Edwards then ducked between some parked cars between them, and he appeared startled when Magaña came up and tried to grab him, the officer said.

The teen then threw a heavy backpack at Magaña, striking his forehead above the right eye.

“The impact of the bag was so great that I was unable to actually see,” he said of momentary loss of sight. “I went black.”

Magaña took a couple of steps before falling on his hands and stomach, discovering the blood gushing down his face and “a pain I’ve never felt before.”

Edwards ran away, and Magaña held onto his backpack until an ambulance took him to Santa Barbara Cottage Hospital.

Magaña said bottle- and rock-throwing started after the injury.

“We’re going to carry you because it’s getting really bad out here,” emergency personnel told him. “People are turning on us.”

Magaña returned for light duty five days later, and said he still has a scar and migraines because of the injury.

Defense attorney Mindi Boulet ran through a frame-by-frame analysis of grainy surveillance footage taken from Sheriff’s Department video cameras.

Magaña said he couldn’t identify Edwards specifically because they were never at close range, and his face isn’t visible on camera.

Boulet asked whether something else had hit him, if he caught the backpack before impact or if he tripped on a small tree stump.

She encouraged the judge to strike testimony relying on Magaña’s cloudy recollection, but Hill denied the request shortly before the end of Magaña’s testimony.

The preliminary hearing is set to continue at 1:30 p.m. Thursday in Department 2.

Noozhawk staff writer Gina Potthoff can be reached at Follow Noozhawk on Twitter: @noozhawk, @NoozhawkNews and @NoozhawkBiz. Connect with Noozhawk on Facebook.