ORANGE, Calif. (NewsNation Now) —  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.

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. It was unclear whether the suspect suffered a self-inflicted wound or was shot by police. He was identified as 44-year-old Aminadab Gaxiola Gonzalez.

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.

The shooting happened at a two-story building around 5:30 p.m. Wednesday. When police arrived on the scene, shots were being fired, Amat said. At least one officer opened fire on the gunman.

Amat had no information about what may have prompted the attack at the building at 202 W. Lincoln Ave. 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.

NewsNation affiliate KTLA was over the scene and footage from the scene showed multiple agencies responding and dozens of officers. Authorities could be seen trying to resuscitate a woman.

Paul Tovar told KTLA 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.”

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.

Rep. Katie Porter, a California Democrat whose district includes the city of Orange, said she was deeply saddened by the shooting.

“I’m deeply saddened by reports of a mass shooting in Orange County, and I’m continuing to keep victims and their loved ones in my thoughts as we continue to learn more,” Porter tweeted. “My team and I will continue to monitor the situation closely.”

The killings were the latest mass shooting in the United States in the last two weeks. Ten people were killed in a Boulder, Colorado, supermarket last week and eight people were fatally shot — including six Asian women — in three Atlanta-area spas the week before that.

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.