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WATCH: Orange Police give update on California shooting that killed 4

Southern California police say the gunman who killed four people and wounded a fifth at an office complex knew all the victims either through business or personally.

Watch the news conference in the video player above.

Orange police Lt. Jennifer Amat also revealed that the gunman had chained gates to the complex closed, forcing officers who responded Wednesday to engage him from outside.

Police withheld the identities of the dead but said one was a 9-year-old boy and the others were a man and two women.

The gunman was also wounded and hospitalized. Wednesday’s shooting happened in the city of Orange southeast of Los Angeles.

The suspect has been identified as 44-year-old Aminadab Gaxiola Gonzalez.

In a tweet, Gov. Gavin Newsom called the killings “horrifying and heartbreaking.”

“Our hearts are with the families impacted by this terrible tragedy tonight,” he wrote.

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U.S. Rep. Katie Porter, a California Democrat whose district includes the city of Orange, said on Twitter that she was “deeply saddened.”

Amat had no information about what may have prompted the attack. She said the shooting occurred on both levels of the building. Signs outside indicated a handful of businesses were located there, including an insurance office, a financial consulting firm, a legal services business and a phone repair store.

People gathered outside the building after the shooting hoping to get word about loved ones.

Paul Tovar told KTLA-TV that his brother owns a business there, Unified Homes, a mobile home broker.

“He’s not answering his phone, neither’s my niece,” Tovar said. “I’m pretty scared and worried … right now I’m just praying really hard.”

Charlie Espinoza also was outside the building and told The Orange County Register that he couldn’t reach his fiancé, who works for a medical billing company.

Cody Lev, who lives across the street from the office building, told the newspaper he heard three loud pops that were spaced out, then three more. There was silence, then he heard numerous shots, followed by sirens and then more shots.

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A Facebook livestream posted by a resident who lives near the office appeared to show officers carrying a motionless person from the building and officers providing aid to another person.

The killings follow a mass shooting at a supermarket in Boulder, Colorado, last week that left 10 dead. A week before that six Asian women were among eight people killed in three Atlanta-area spas.

The city of Orange is about 30 miles (48 kilometers) from Los Angeles and home to about 140,000 people.

Amat said the shooting was the worst in the city since December 1997, when a gunman armed with an assault rifle attacked a California Department of Transportation maintenance yard.

Arturo Reyes Torres, 41, an equipment operator who had been fired six weeks earlier, killed four people and wounded others, including a police officer, before police killed him.