Authorities announced 22-year-old Brandon Colbert, of Tulsa, Oklahoma, was arrested for the murders of Carina Mancera and Jennabel Anaya.
Police said Colbert opened fire on Mancera, Luis Anaya and their child, Jennabel Anaya, after they returned from grocery shopping shortly before 9:30 p.m. on Aug. 6.
Mancera was pronounced dead at the scene, and the toddler was taken to a hospital where she died. The father was not hurt.
On Oct. 5, authorities identified as a suspect, a man who had been previously sought as a person of interest in the case.
Investigators released surveillance video of Colbert, who they said boarded a Blue Line train five blocks from the crime scene shortly after the shooting.
Authorities stated Colbert's DNA was found on items at the scene of the double murder. His DNA had already been in their system due to a recent arrest for driving a stolen car, according to police.
"After his arrest by the Los Angeles Police Department, his DNA was collected and submitted through that system. Last Monday was when we were notified," Lt. Lloyd Cox with the Long Beach Police Department said.
A $40,000 reward was offered for information leading to the gunman's arrest and conviction.
No motive has been given in the killings.
Police were still urging those with information about Colbert to come forward. Those with information that could aid detectives in their investigation were urged to call the Long Beach Police Department at (562) 570-7244.