"The first reason is believed to be related to the business and personal relationship that existed between the suspect and all the deceased," said Orange police spokesman Lt. Jennifer Amat, at a press conference on Thursday.
"This was not a random act of violence," he said.
Suits in which some victims were found with a business known as Unified Homes, he said, a real estate business focused on mobile homes.
Amat did not identify the victims, saying they had not yet completed the relatives' notices. Among the dead were two women, a man and a nine-year-old boy, he said.
The boy, found in a complex, was believed to be the son of one of the victims who worked for the business, Amat said. A fifth victim, a woman, was found with the boy. He was taken to a local hospital, where he was in a critical but stable condition, Amat said.
Police identified the suspect as Aminadab Gaxiola Gonzalez (44). He returned to a local hospital in a critical but stable condition, Amat said.
Investigators found a handgun and a backpack containing pepper spray, handcuffs and ammunition, which officials believed to be the suspect's.
Four people, including a child, were killed in a shooting at an office in Orange, California, on Wednesday, a police official said.
At least 20 shootings have taken place in the two weeks since the Atlanta spa train attack left eight dead.
The fifth victim was injured and taken to a local hospital in critical condition, said Orange Police Department spokesman Lt. Jennifer Amat at a press conference.
Police responded at around 5.30pm. then encountered a gunshot when they arrived at the scene, Amat said. Police arrested the suspect on charges and "there was a shooting incident," he said.
The suspect was taken to hospital after being shot, Amat said. It was not clear if the suspect's injuries were his own, he said.
One firearm was found at the scene and the suspect was in a critical condition, he said.
The department did not provide further details about the suspect or the victims. Officials are expected to hold a press conference at 10:00 am PT / 1:00 pm ET to release new information.
Investigators gathered outside the office building where the incident took place in Orange, California, on Wednesday, March 31, 2021.
Paul Tovar told CNN affiliates KCBS / KCAL near the scene that he had not heard of his brother or nephew, both of whom worked at the building.
"I'm just trying to find his well-being," Tovar said. "She is not answering her phone, she is also my nephew. I am very scared and worried. I wish I had known more."
Tovar told CNN later on Thursday morning that he had no news of his brother or nephew.
Orange is located about 30 miles south of Los Angeles. The building where the incident took place has several businesses including Farmer's Insurance, Calco Financial and Miller Counseling.
It was not immediately clear where the incident took place in the building.
'It's scary and painful'
The Los Angeles FBI has confirmed to CNN that it has responded to the shooting as usual, but the Orange Police Department is in the forefront of the investigation unit.
This is at least 20 shootings since it hit an area in Atlanta two weeks ago that left eight people dead. CNN describes the mass shooting as a shooting incident that results in the death of four or more people (dead or injured), other than the shooter.
"I can tell you that we have never had an incident like this in the city of Orange since 1997," Amat said. "It is such a tragedy for the victims, their families, our community and our police department."
"It's horrible and heartbreaking. Our hearts go out to the families affected by this terrible tragedy tonight," said Govin Newsom, California.
Katie Porter, a California Democrat representing Orange County, said she and her staff were monitoring the situation.
"I am deeply saddened by the reports of the mass shooting in Orange County, and I continue to keep the victims and their loved ones in my thoughts as we continue to learn more," Porter said in a tweet.