Christianson didn't say when the 33-year-old came to the US but said that the Mexico native has been living here for several years.
Ahead of the congressional elections in November, a TV campaign ad endorsed by Trump focused on an undocumented Mexican immigrant convicted of killing two sheriff's deputies in California.
Christianson spoke about making laws stricter as Singh's brother wept beside him. Singh leaves behind a wife and a five-month-old son.
Richardson said he appreciated the many tips to investigators into Singh's shooting, but he lamented that people want to remain anonymous in these situations. Singh joined the force in 2011 and was one of just 12 members of the Newman Police Department. Ronil Singh was gunned down early Wednesday. "And under SB54 in California, based on two arrests for DUI and some other active warrants that this criminal has out there, law enforcement would've been prevented, prohibited from sharing any information with ICE about this criminal gang member".
The suspect also had gang affiliations, Christianson said.
President Donald Trump has also weighed in on Twitter, referencing Singh's killing and writing: "Time to get tough on Border Security".
Ronil Singh of the tiny Newman Police Department on Wednesday after being stopped on suspicion of drunken driving.
Trump tweeted about the shooting on Thursday, days after the December 22 partial shutdown of the federal government. "It's a conversation we need to have".
The sheriff leading the investigation into the shooting death of a California police officer took aim at so-called sanctuary laws that limit cooperation with US immigration authorities. It includes more than 800 exceptions for violent crimes and felonies and bars police from asking people about their citizenship status.
Singh also was an immigrant, coming legally from his native Fiji to fulfill his dream of becoming a police officer, authorities said.
Ronil Singh are: Gustavo Perez Arriaga, who turned 32 on Friday; his brother, Adrian Virgen, 25; and co-worker Erik Razo Quiroz, 32.
Governor Jerry Brown has said the law strikes a balance between protecting families and ensuring consequences for serious criminals.
In a press conference on Thursday, Christianson said, "We will relentlessly continue to hunt our suspect down and bring him to justice so that ... we can bring closure onto this community and this police department".
Christianson stated the suspect was not a resident of Newman and had immigrated into the United States illegally. This is a man that worked hard for what he believed in.
Newman Police Chief Randy Richardson stepped in to the answer that question.