Authorities had previously announced the arrest of two men, both described as gang members, in the series of shootings, and initially said a third suspect was still at-large.
Now they say that third individual is in custody as well. He was described only as a male juvenile who was arrested in Compton Thursday night. No other details about the suspect, his alleged role in the killings or possible gang affiliations were disclosed.
The juvenile's case will be presented to the Los Angeles County District Attorney's Office to consider filing of charges.
The DA's Office has already announced charges against the two adult suspects - Gary Garcia Jr., 42, and Timberland Wayne McKneely, 20, - including four counts of murder and two counts of attempted murder.
The shooting spree happened over a span of just three hours Sunday night into Monday morning within a five-mile radius in Bell, Huntington Park, Cudahy and unincorporated areas of L.A. County.
Details on the victims and the shootings here.
2 alleged gang members charged in 'random and brutal' LA County shooting spree that left 4 dead
In the meantime, a vigil was held Friday to honor the memory of the youngest victim, 14-year-old Javier Pedraza Jr. of Cudahy.
The somber gathering was held at Ellen Ochoa Learning Center, near where the teen was gunned down.
At the vigil, one community activist said more needs to be done to combat the violence.
"You have to put cameras on the streets, you have to put shot-spotters on the streets," said Joseph Trevizu. "There are countless shootings people don't know about."
A GoFundMe page has been established to help Javier's family.