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

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

Police recovered an automatic handgun and a backpack with pepper spray, handcuffs and ammunition.

Police withheld the identities of the dead but said one was a nine-year-old boy. The others were a man and two women. The wounded victim was only identified as a woman.

The suspect, identified as Aminadab Gaxiola Gonzalez, 44, was also wounded and taken to hospital. It was unclear whether he suffered a self-inflicted wound or was shot by police.

Wednesday’s shooting happened in the city of Orange south east of Los Angeles. When officers arrived, shots were ringing out at the building.

Police said the shooting involved the office suites of a company called Unified Homes, a mobile home brokerage.

Paul Tovar told KTLA-TV that his brother owns Unified Homes.

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

Ms Amat said the shooting happened on both levels of the building.

Signs outside indicated a handful of businesses are 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.

Charlie Espinoza was outside the building and told The Orange County Register that he could not reach his fiancee, who works for a medical billing company.

The violence was the third US mass shooting in just over two weeks.

California Governor Gavin Newsom called the killings “horrifying and heartbreaking”.

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

The killings followed 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 at three Atlanta-area spas.

“Enough is enough,” US senator Dianne Feinstein, from California, tweeted. “We have to do something about the guns on our streets.”

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 a period of silence and then Mr Lev said he heard numerous shots, followed by sirens and more shots.

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 helping another person.

Orange is about 30 miles from Los Angeles and home to about 140,000 people. Ms 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.