Karine Hakobyan, 38, was shot to death on March 26 in a car port at her apartment complex in the Little Armenia area.
Alberd Tersargyan was arrested Wednesday night and charged with one count of murder with the special circumstance of lying in wait, which could make him eligible for a death sentence.
Police allege he waited for Hakobyan to come home, and then shot her in the head. Police said ballistics tests on a handgun they recovered link him to the killing.
Police are looking into the possibility that the suspect is also responsible for the 2008 murders of Hakobyan's husband 43-year-old Khachik Safaryan and their 8-year-old daughter Lusine.
Their bodies were found in their home by the couple's then-12-year-old daughter when she came home from school in December 2008. They had been shot in the head.
Months after the killings and before her mother's murder, the teen composed a letter to Obama that was never sent.
"Our family is falling apart now. My grandparents are sick and depressed. My mother cannot cope with the loss of her husband and daughter," she writes in the letter.
"I am not in any better shape. I still see the bloody bodies of my sister and my father as I found them that day."
After she found her mother's body, she added a P.S. to the letter.
"When I arrive in front of my mom's wide open car door, I see blood. Slowly, I feel the world tear down in front of my eyes. Every second, thousands of things cross my mind. I get closer, panicking, shaking my mom twice, running to get help, yelling," she wrote.
The teen wrote the letter as an appeal for help bringing her aunt over from Armenia. The letter has yet to be sent.
Tersargyan made a brief appearance at a downtown L.A. courthouse, but his arraignment was postponed until May 6. He was ordered to remain jailed without bail.