A man in his 80s apparently shot two women shortly after noon before turning the gun on himself in the lobby of the Golden West Tower apartments in Torrance, said Sgt. Robert Watt.
One of the women was an employee of the apartment complex, but victim identities are being withheld until next of kin is notified, Watt said.
Witness Arturo Ramirez, who works at the Golden West Towers, told the Los Angeles Times he locked himself in an office to call 911 after seeing a man who lived in the facility holding a gun.
Ramirez said he heard one of the women killed say, "Please, no! Please!" and then heard two gunshots. A final gunshot came a few minutes later.
Police are still investigating the man's motive, but residents and staff from the senior center told NBC4 that the man had been behaving erratically, and many of them were seeking to have him kicked out of the complex.
"People knew that he was a little bit off," Golden Tower employee Alex Galindo told the TV station. "People were warned, but nobody did anything, so I know there's going to be blood on a lot of people's hands. That's all I can say at this point."
A handgun was recovered at the scene.
Watt said the building's management had received complaints about the alleged shooter but police were never alerted.
Police were trying to obtain surveillance camera recordings of the shooting, he said.
According to the city of Torrance's website, Golden West Towers is a 180-unit, privately owned and operated facility. The low- and moderate-income housing is for seniors who can live independently.
A call to building management wasn't answered Tuesday night, and the voicemail box was full.
The incident was the latest in a string of murder-suicides at senior centers in Southern California in recent years.
In a shooting last month, investigators suspect 45-year-old Colin McGrattan shot and killed his ex-wife and her two nieces at O'Connor Woods Senior Living center in Stockton before killing himself.
In 2010, 77-year-old Arthur Baar shot and killed his wife, Caryl, 72, before turning the gun on himself in Laguna Woods, the south Orange County retirement community formerly known as Leisure World.