San Diego police have said the gunman, who shot seven people at a pool party on Sunday (30 April), was on the phone with his ex-girlfriend during the shooting spree.
Peter Selis, 49, resident of the same luxury apartment complex where the incident occurred, was reportedly depressed after his relationship with his girlfriend ended.
According to reports, Selis opened fire at the people at La Jolla Crossroads Apartments, killing one woman and injuring six others before being shot dead by police.
The victims of the attack included four black women, two black men and a Hispanic man. Authorities maintain the attack was not racially motivated, and that it is doubtful that Selis, who was white, knew any of the approximately 30 people at the party.
"These victims were just in his presence at the wrong place at the wrong time," San Diego Police Chief Shelley Zimmerman, dismissing reports of hate crime, said on 1 May.
Selis was sitting beside the pool on his lounge chair when he opened fire. According to the police, he called his ex-girlfriend after shooting two people and made her listen to the assault.
"It is apparent that Selis wanted his ex-girlfriend to listen in as he carried out his rampage," Zimmerman said.
She added that from the information gathered so far "it is very clear that Selis was very despondent over the breakup of his girlfriend".
Selis' family has also confirmed that he was "distraught and depressed" over the breakup, but, added that they had no idea that he would react in a violent way, the BBC reported.
Meanwhile, one witness of the attack said that Selis was apparently drinking beer when he opened fire, but police have not confirmed it. While, another witness said Selis first told people "to leave" the area before he began shooting.