Jairo Saenz pleaded guilty to racketeering and weapons charges six months after his brother, Alexi, pleaded guilty to eight murders.
"I knew what I was doing and I knew I was wrong," Saenz told the judge. "I came to the U.S. and worked for two years. Then I went to the streets and joined the MS-13 gang."
Saenz said he acted as the driver to many of the shootings.
The bodies of Cuevas and Mickens were found bludgeoned to death in September 2016. Prosecutors said they had been killed following a dispute at their high school.
Mickens' mother Elizabeth Alvarado was there in court on Tuesday.
"Every parent, every mother, anybody they're always wanting to know why? Why did it happen, why did you choose my daughter that day or anybody's kids that day?" Alvarado said.
She said her daughter loved playing basketball and wanted to study medicine.
"Sometimes I'll just you know, I'll sit there and look at her urn and be like, why are you there and not with me," Alvarado said.
That's the same pain shared by George Johnson, who lost his young son Michael in a similar murder in January of 2016 in Brentwood.
"The holidays get rougher and rougher, I got to look at all my other kids that are there, when they're there, that's when I feel it," Johnson said.
Prosecutors say at the time of the murders, Jairo Saenz was second in command in the gang to his brother Alexi Saenz.
"It should've never happened, he's not God, he has no power to take a life just like that with no remorse," Alvarado said.
Jairo Saenz faces 40 to 60 years behind bars when sentenced on June 13.
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