The man suspected of killing a police officer early Wednesday in Stanislaus County, California, is in the country illegally, authorities said Thursday. Stanislaus County Sheriff Adam Christianson did not publicly identify the man suspected of shooting Corporal Ronil Singh during a traffic stop, but said they know who he is and they are "pursuing every investigative lead."

Singh, 33, was originally from Fiji and had been with the department for more than seven years. He leaves behind a wife and young son.

"This suspect, unlike Ron, who came to this country lawfully and legally, to pursue public service and being a police officer, this suspect is in our country illegally," Christianson said. "He doesn't belong here. He's a criminal. We will find him, we will arrest him, and we will bring him to justice."

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Christianson said they have "reason to believe" the suspect is still in Stanislaus County.

The Stanislaus County Sheriff's Department tweeted several more photos of the suspect late Thursday.

The following pictures of the suspect including a name & birth date have been circulating on social media. We can confirm these are pictures of the man who murdered Corporal Ronil Singh. We CANNOT, however, confirm that the correct name or birth date is being put out. pic.twitter.com/cSCvE1DhEF — Stanislaus Sheriff (@StanSheriff) December 28, 2018

Singh was shot and killed after making a stop as part of a DUI investigation just before 1 a.m. Wednesday in Newman, California. Christianson said Thursday there was a "gunfight" between Singh and the suspect.

"Corporal Singh absolutely tried to defend himself and stop this credible threat," Christianson said.

Photo provided by the Newman Police Department shows officer Ronil Singh, who was shot and killed during a traffic stop early Wednesday, Dec. 26, 2018, in Newman, Calif. Stanislaus County Sheriff's Department via AP

Police on Wednesday released surveillance video of the suspect buying several packs of beer at a convenience store before getting back into his Dodge Ram pickup truck. It's believed the suspect stopped at the convenience store before the shooting. Police said they have located and seized the pickup truck.

Newman's Chief of Police Randy Richardson, who said he relieved Singh on Christmas Day, fought back tears as he described his colleague.

"When I sat with him in the chief's interview, he told me he came to America to become a police officer," Richardson said. "That's all he wanted to do. He truly loved what he did. You've never seen a man smile more than him."