Patrice Williams thought she was hearing the pops of balloons she had inflated earlier as she gathered with friends and family for cake to celebrate her daughter turning two. But the loud bursts ringing through the banquet hall were actually the sounds of gunshots that ultimately killed four guests at the birthday party she had thrown.

“I don’t know what happened, and I’m just so shocked and lost,” Ms Williams told the Associated Press.

The mass shooting in Stockton, California, over the weekend killed three children aged eight, nine and 14, and a 21 year-old, authorities said. Eleven others were injured, one critically, in the shooting where about 100 guests were celebrating.

Ms Williams’ other daughter, a cousin and three of her friends were among those shot while party goers gathered around a cake, she told the AP.

Others were about to sit down for dinner in the banquet hall when the shooter – or shooters – entered with an automatic weapon. Authorities believe the attack may have been targeted.

The shooting appeared to begin inside the event space before moving onto the street and it’s unclear if anyone returned fire, authorities have said.

Ms Williams said she didn’t see the shooter and has no idea who would commit such violence.

“They deserve to be in jail. They deserve to go to hell,” she told AP. “I’m sorry, but I just … it’s not respectable. It’s a kids’ party.”

Later that night, sirens from ambulances and police cars wailed through dense fog, keeping residents awake as police searched for perpetrators and for clues about the crime, the Los Angeles Times reported.

The once-festive locale – replete with rainbow-hued balloons and a swan-shaped bounce house – is now a crime scene being investigated by the San Joaquin Sheriff’s Department, FBI and other federal agencies.

Sitting about 80 miles (130 km) east of San Francisco, Stockton has been seeing a rise in violent crime, including homicide, rape, assault and robbery. The broader San Joaquin Valley, which also encompasses smaller neighbouring towns, had the highest violent crime rate in the state in 2023, according to the Public Policy Institute of California.

San Joaquin County Sheriff Patrick Withrow said authorities were “confident that this was not a random act”.

They believe multiple shooters were involved, who are still at large. While the city of 320,000 is gripped by fear, officials have said there is no continuing threat to the community.

They are calling on the public to provide tips and videos and flesh out rumours relating to the shooting. They are also offering a growing reward for information.

Stockton Mayor Christina Fugazi said in a statement that the perpetrators would be caught and “you will never see the outside of a prison cell again”.

“If there is even a flicker of humanity left in you, turn yourselves in immediately,” she said.

While the mayor suspects gang violence was the motive, Sheriff Withrow says it is too early to say.

On Sunday evening he said the community needed “to show that we will not put up with this type of behaviour, when people will just walk in and kill children” by coming forward to “tell us what you know”.

On Monday, California Governor Gavin Newsom ordered flags at the state capitol in Sacramento to be flown at half-staff “to honour the children we mourn and stand with the community as it grieves”.

While the victims’ names have not been officially released, family and friends have begun identifying some of them online and in the media.

Maya Lupian, “the most outgoing eight-year-old you’d ever meet”, was identified by her sister Yesenia as one of the deceased on a crowdfunding site.

She excelled in academics and extracurricular activities, had a purple belt in karate and loved to dance, sing and draw,

“Maya was the light in everyone’s life, the sweetest soul,” her sister wrote.

The 14-year-old who was killed has been identified by his family as Amari Peterson, a “bright, loved, and promising young soul whose life was taken far too soon by a senseless act of violence”.

From the neighbouring city of Modesto, he was an American football and basketball player with plans for college, his family wrote on a crowdfunding website seeking donations for his funeral.

“He was simply being a kid at a kids’ party,” they wrote. “No family should ever go from planning birthdays to planning funerals.”

The boy’s father, Patrick Peterson, told the Sacramento Bee that he “can’t live with the fact that I couldn’t save my son”. His nine-year-old daughter was also injured, but escaped by fleeing from a back doorway and climbing a fence to safety, he said.

One man, Emmanuel Lopez, told the LA Times that his brother Susano Archuleta was the 21-year-old who was killed, saying he had been hit in the neck and died at the scene.

“I’m just upset right now. My brother died in my arms,” Mr Lopez said, describing Mr Archuleta as “full of life and energy”.

Family members, some still wearing their party clothes, returned to the tragic scene the next day to retrieve their cars and possessions, according to the LA Times. They declined to share their names out of fear that the shooter or shooters would find and hurt them.

“Who thinks this would happen at a baby birthday party,” a woman who identified herself as a great-aunt of the birthday girl said. “This don’t make no sense. A mass killing is that this was. Senseless. Crazy. Find the people who did it.”

The event space – formerly a children’s theatre that is also used for content creation and a recording studio – is unlikely to reopen soon, if at all, its tenant Willie Colins told the Sacramento Bee.

“There’s no business there anymore,” Mr Colins said. “It’s a memorial.”