The adult suspect was also arrested for the killing of a 45-year-old man days after the teen victim's death. The two suspects are believed to be responsible for as many as six homicides, Los Angeles Police Chief Michel Moore said at a news conference.
Details of the other four killings were not provided.
Alfonso Ramos, 16, was fatally shot shortly after 5 p.m. Aug. 12 at the Evergreen Park Recreation Center, in the 2800 block of East Second Street, investigators said.
One of the suspects in that shooting was identified as 26-year-old Michael Monarrez, the LAPD said in a statement. "The second suspect was identified as a 14-year-old juvenile. Detectives were making investigative efforts to locate the pair and effect an arrest, when a second incident occurred."
At about 10 a.m. on Aug. 17, Hollenbeck Division officers responded to the 500 block of Lorena Street for a shooting investigation.
Police arrived to find the victim, later identified as 45-year-old Fermin Ruelas, suffering from multiple gunshot wounds. Ruelas died at the scene, according to the L.A. County coroner's office.
Monarrez was arrested as a suspect in Ruelas' killing a short time afterward, and on Aug. 18 investigators arrested the 14-year-old boy they say was involved with Monarrez in Ramos's killing. The juvenile suspect was not publicly identified due to his age.
A GoFundMe account has been set up to help Alfonso Ramos' family. His mother said he worked at the Boyle Heights recreation center as a volunteer at Summer Night Lights. She says he touched many lives with his sense of humor and determination to better himself.
Anyone with information regarding the case is urged to call Detectives Riojas or Calzadillas of the LAPD's Central Bureau Homicide unit at (213) 486-8700.