The incident was reported just before 7 p.m. at a home in the 11600 block of Lerdo Avenue.
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Officers made their way inside after no one answered the door and were met by a witness who directed officers to the victims.
According to investigators, that witness barricaded themselves in a room during the shooting and called police, but details on who that person is were not immediately released.
Read more: Lone survivor of Granada Hills murder-suicide was daughter of suspected gunman
"I don't know how much more terrifying and horrific of a scene it can be when you have four persons that are deceased," said LAPD Capt. Kelly Muniz during a press conference Saturday night. "The only positive point is that you at least have one witness that has survived this incident."
The suspect, who was described only as a man in his 80s, fatally shot his wife, who was also in her 80s, and their two adult children, a man and a woman in their 40s. He was found with a self-inflicted gunshot wound in a different part of the house, police said.
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A neighbor who stopped by the scene was shocked to hear what unfolded near his home, saying he never got the opportunity to meet the victims.
The incident remains under investigation. The area near Lerdo Avenue remained closed off for most of the night.