Lazarus was arrested on suspicion of murdering her ex-boyfriend's wife, Sherri Rae Rasmussen, who was shot to death in 1986.
Lazarus allegedly had a relationship with John Ruetten, Rasmussen's husband. Ruetten discovered Rasmussen's body, beaten and shot multiple times, on February 24, 1986, in the Van Nuys condominium he shared with Rasmussen.
LAPD investigators at the time thought Rasmussen had been killed by armed robbers.
Undercover officers obtained a DNA sample from Lazarus without her knowledge, that matched the killer's DNA, resulting in Lazarus's arrest Friday.
Lazarus's DNA was legally collected from "discards," police said at a Friday afternoon press conference, without elaborating. Police have previously used techniques like tricking suspects into leaving DNA on objects like cups or cans.
The Associated Press contributed to this report.