The shooting in Orange, California, that killed four people - including a nine-year-old boy - was carried out by a 44-year-old man who knew the victims, police said as they ruled out the attack as a random act.
The other fatalities included a man and two women, Orange Police Department Lieutenant Jennifer Amat told a news conference.
Another woman and the shooter were hospitalised, Ms Amat said.
Officials identified the suspect as a resident of the nearby city of Fullerton, saying he entered a business called Unified Homes yesterday afternoon and locked the gates behind him with a bicycle lock.
Authorities recovered a semi-automatic handgun, pepper spray and handcuffs.
The bloodshed in the city of Orange, about 48km southeast of downtown Los Angeles, marked the third deadly mass shooting in the United States in less than a month.
California governor Gavin Newsom posted on Twitter: "Horrifying and heartbreaking. Our hearts are with the families impacted by this terrible tragedy tonight."
"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," US Representative Katie Porter from California also tweeted.
It comes after two other high-profile mass shootings earlier this month, which set off a renewed debate about gun control measures in the United States.
On 22 March, 10 people were killed in a shooting at a grocery store in Boulder, Colorado, less than a week after a man shot and killed eight people, including six women of Asian descent, at spas in Atlanta, Georgia.
