Stanislaus County Sheriff Adam Christianson, who led the investigation, blamed California's sanctuary law for preventing local authorities from reporting Perez Arriaga to USA immigration officials for deportation after two previous drunken driving arrests.
The Stanislaus County Sheriff said Singh was conducting a traffic stop for a suspicion of driving under the influence when the shooting occurred.
The Stanislaus County Sheriff's Department, which has been leading the investigation, shared on social media that Arriaga will be charged with homicide.
The killer is identified as Gustavo Perez Arriaga, an illegal immigrant from Mexico who was in the USA after crossing the Arizona border.
It was not immediately clear if Arriaga had an attorney. "We were prohibited, law enforcement was prohibited because of sanctuary laws, and that led to the encounter with Officer Singh".
The sheriff says Perez Arriaga had two prior drunken driving arrests and known gang affiliations.
"Why are we providing sanctuary for criminals?" he asked. "We need closure, his family needs closure".
Christianson blamed California's sanctuary law for preventing local authorities from reporting Gustavo Perez Arriaga to federal immigration officials.More news: Ousted PM Nawaz Sharif Sentenced for Corruption
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President Donald Trump used the shooting and Arriaga's status to push his immigration policies.
The officer, identified as 33-year-old Ronil Singh, was a native of Fiji who immigrated to the United States to become a police officer and rose to the rank of corporal, authorities said.
Two other men, Adrian Virgen, 25, and Erik Razo Quiroz, 27, were arrested December 28 for accessory after a felony for trying to divert investigators, Christianson said, according to NBC News.
Trump, during his campaign for president and in the White House, has denounced a number of crimes by illegal immigrants to press for stricter immigration enforcement.
His younger brother, Reggie Singh, attended the news conference on Friday, crying emotionally at the podium and thanking police profusely for the arrests.
"He came to America with one objective, and that was to serve this country", said Newman Police Chief Randy Richardson, calling Singh an "American patriot". Christianson said the case was being handed over to a prosecutor, and he suggested the killing could be a capital murder case. He called the 33-year-old with a newborn son an "American patriot".
He was a reserve officer for the Merced County Sheriff and worked as an animal control officer in Turlock before he joined the Newman force.
"Our hearts are with the entire community of Newman and law enforcement officers across the state who risk their lives every day to protect and serve the people of California", Brown said. He had a thick accent but took speech classes to improve his communication, the chief said.