Five shots at the Oakland Coliseum during A’s post-game fireworks display

Five people were hit by stray bullets during a July 4 fireworks show at the Oakland Coliseum in what police say was a burst of “celebratory” gunfire from somewhere outside the stadium.

Three fans were treated for injuries caused by bullet fragments on Monday night after crowds gathered on the pitch for a post-game fireworks display. The shooting began around 9:30 p.m., officials said.

A fourth victim later went to a local hospital, and on Tuesday a fifth victim called police. Police have released few details about the victims other than to say none of the wounds were life-threatening. The youngest victim was 12, a law enforcement source said.

An initial ballistics analysis showed at least some of the bullet fragments came from a high-powered rifle, Chief LeRonne Armstrong told reporters at a Tuesday news conference.

The shots came at a happy moment for A’s fans after the team defeated the Toronto Blue Jays 5-1 on Monday night, drawing 24,403 fans. It also capped a tumultuous Oakland holiday weekend when the city witnessed three homicides, including a murder-suicide in the Adams Point neighborhood, Armstrong said.

As of Tuesday, Oakland had recorded 55 homicides that year, plus two more homicides. While there have been no injuries from stray bullets at previous fireworks shows at the Coliseum, the loud shooting on July 4 was a long-standing problem, Armstrong said.

“If a bullet is fired into the air, eventually that bullet will come down” and fall at a slower rate than when it was launched, although it’s still a powerful projectile, Armstrong said.

“That’s why we warn people against firing guns indiscriminately in the community,” he said.

Officials have searched the Colosseum and believe the bullets were fired outside the venue – possibly as much as a mile away – given the number of calls they received that night about gunfire in the area.

Oakland was also rocked by several shootings over the weekend, one of which seriously injured a 3-year-old. Police are investigating a relative of the child in this case, Armstrong said.

John Jones III, an Oakland violence prevention worker who attended the game with his son, said security guards temporarily blocked a ramp leading to the pedestrian walkway to the BART Coliseum station, causing a bottleneck as fans exited the venue. Guards cleared the way after about 10 minutes, Jones said. He saw no law enforcement and said no one informed the crowd that a shooting had taken place.

“It wasn’t a shooting,” said Jones, who has witnessed the aftermath of many violent incidents in Oakland. Three other attendees interviewed by The Chronicle said they saw nothing out of the ordinary except for an abundance of rogue fireworks.

Police are offering a reward of up to $20,000 for information leading to an arrest. Anyone with tips can call the department’s personal injury unit at 510-238-3426. In its release, the department pledged to continue working closely with the A’s security to ensure the safety of everyone participating in future games.

Speaking to reporters ahead of Tuesday’s game against the Blue Jays, A manager Mark Kotsay urged fans not to be alarmed and said he considers the Colosseum to be “a safe environment”.

On July 4, “people tend to do things that aren’t normal,” Kotsay said, adding that it was “really unfortunate” to see such recklessness near the Colosseum.

Chronicle contributor Matthew Kawahara contributed to this report.

Rachel Swan is a contributor to the San Francisco Chronicle. Email: Twitter: @rachelswan