One of four people shot dead in a Los Angeles real estate office was a 9-year-old boy who apparently died in the arms of his wounded mother as she tried to shield him.
Police also say the suspected gunman in Wednesday's attack, who was himself wounded before being taken into custody, knew his victims and had a family relationship with at least one.
The bloodshed inside a two-storey office building in the city of Orange, about 48km southeast of Los Angeles CBD, marked the third deadly mass shooting in the United States in a month.
But the suspect's connection with those killed appeared to set it apart from the shootings in Georgia on March 16 and Colorado on March 22, in which the victims were believed to have been slain at random.
"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 Police Department Lieutenant Jennifer Amat said.
The identities of the victims were being withheld pending notification of relatives, police said.
The surviving victim, a woman badly wounded while cradling her son as he was fatally shot, worked at the business.
She remained in hospital in a critical but stable condition on Thursday.
Three other adults - two women and a man - were killed.
The suspect, identified as Aminadab Gaxiola Gonzalez, 44, a resident of nearby Fullerton, also was listed in critical condition.
He may be arraigned on murder charges from his hospital bed on Friday, Orange County District Attorney's office said.
He is accused of opening fire in the office of a mobile home realty company after padlocking entry gates to an interior courtyard of the building.
Police officers, encountering gunfire as they arrived were forced to use bolt cutters to open them.
"In the meantime, a horrific rampage was going on in offices and people were dying or were being shot," District Attorney Todd Spitzer said.
From outside, two of the officers engaged in a gunfight with the suspect, who was wounded and arrested, Amat said.
Investigators, who recovered a semi-automatic handgun from the scene, are yet to conclude whether the gunshot he suffered was self-inflicted or fired by police.
A suspect fatally shot this week by Chicago police in what the department called an "armed confrontation" jas meanwhile been identified as a 13-year-old.
Adam Toledo of Chicago died of a gunshot wound to his chest in the early hours of Monday, Cook County Medical Examiner's Office said on Thursday.
Chicago Police said an officer was responding to a scene where gunfire had been detected when the officer shot the suspect.