The gunman in a mass shooting that claimed the lives of four Californians including a nine-year-old child personally knew and targeted his victims in a premeditated attack, police said Thursday.
The attack at a small office building in the city of Orange, south of Los Angeles, Wednesday evening followed two mass shootings earlier in March that once again reignited the debate on gun control in the United States.
"This was not a random act of violence," said police lieutenant Jennifer Amat at a press conference.
"The preliminary motive is believed to be related to a business and personal relationship, which existed between the suspect and all of the victims, and it appears all of the adults were connected either by business or a personal relationship," she said.
The suspect, identified as 44-year-old Aminadab Gaxiola Gonzalez, had closed access gates to the building with bicycle locks which delayed police entering the site.
Police opened fire on the gunman through the gates, seriously wounding him, and the suspect remains in hospital.
On entering the building, police found the dead child and an adult woman believed to be his mother, who is in critical condition.
"It appears that a little boy died in his mother's arms as she was trying to save him during this horrific massacre," said district attorney Todd Spitzer.
The three other victims -- two women and a men -- were found at a small real estate company office and on a nearby landing.
A semiautomatic pistol and a backpack containing pepper spray, handcuffs and ammunition thought to belong to the gunman were retrieved at the scene.
The latest US mass shooting followed the killing of eight people at spas in Atlanta, Georgia and the murder of 10 people at a supermarket in Boulder, Colorado, both in March.
