The gunman who killed four people, including a 9-year-old boy and three adults at a business park in a Los Angeles suburb, had a "business and personal relationship" with his victims, police said Thursday. 

"The preliminary motive is believed to be related to a business and personal relationship which existed between the suspect and all of the victims," Orange City Police Lieutenant Jennifer Amat said, adding, "This was not a random act of violence." 

Police said they received multiple calls around 5:30 pm local time Wednesday of shots being fired at a business park in Orange, around 30 miles southeast of Los Angeles.

When officers approached the complex, they discovered that the alleged shooter, Aminadab Gaxiola Gonzalez, 44, had locked the wrought-iron gates from inside using "a bicycle type cable lock", forcing the police to engage the suspect from outside the premises.

A shootout ensued before police were able to force their way into the building using bolt cutters. Upon entering, they located the injured suspect and took him into custody.

It's unclear whether Gonzalez, identified as a resident of Fullerton, California, sustained his injuries from a self-inflicted gunshot or from the exchange with police.

Police also found two victims at the courtyard, one of whom was the 9-year-old boy. He was found in the arms of a woman who officials believed to be his mother. She was shot while trying to protect him, according to Orange County District Attorney Todd Spitzer.

The woman was transported to a local hospital, along with the suspect. They remained in critical but stable condition on Thursday.

Police found three other victims in the complex upon entering, Amat said, including one woman on an upstairs outdoor landing, one man inside an office and one woman inside a separate office. Police withheld the identities of the victims, pending notification of next of kin. 

The suite where the victims were located has been identified as Unified Homes, a real estate business specializing in mobile homes. 

Signs outside the building indicated several types of businesses were located there, including an insurance office, a financial consulting firm, and a legal services business.

Authorities said they recovered on the scene a semiautomatic handgun, a backpack containing pepper spray, handcuffs and ammunition, all of which belonged to the suspect. 

Spitzer said Thursday that the suspect could face the death penalty, because the incident is a special case with multiple victims. 

"And we are now presently looking at whether his locking of the front and the back gates constituted a lying in wait, which would also be eligible for death," he added. 

The Wednesday mass shooting occurred just more than a week after a 21-year-old gunman opened fire at a Boulder, Colorado, supermarket, killing 10 people, including a police officer.

That incident, which took place on March 22, followed a March 16 massacre at three Atlanta-area spas during which eight people, including six Asian women, were killed. 

"Horrifying and heartbreaking. Our hearts are with the families impacted by this terrible tragedy tonight," California Governor Gavin Newsom said on Twitter on Wednesday evening.

US Representative Katie Porter, a Democrat whose congressional district includes Orange County, said she was "deeply saddened by reports of a mass shooting in Orange County". 

"I'm continuing to keep victims and their loved ones in my thoughts as we continue to learn more," she added. 

Authorities said it was the city's deadliest shooting since 1997, when a former employee of the California Department of Transportation killed four people and wounded two others with an assault rifle.