Authorities said they identified but won't yet name the man who killed Singh of the 12-person Newman Police Department on Wednesday and has not been captured. They believe the attacker is still in the area some 100 miles southeast of San Francisco and is armed and dangerous.
"This suspect is in our country illegally. He doesn't belong here. He is a criminal," Stanislaus County Sheriff Adam Christianson, whose agency is leading the investigation, told reporters.
Newman Police Chief Randy Richardson fought back tears as he described Singh, a 33-year-old with a newborn son, as an "American patriot."
"He came to America with one purpose, and that was to serve this country," Richardson said.
VIDEO: 'Please remember the man': Police chief gives emotional statement, calling slain Newman Cpl. Ronil Singh an 'American patriot'
Singh drove more than two hours each way to attend the police academy in Yuba City, Richardson said. He joined the Merced County Sheriff's Office as a reserve officer and worked as an animal control officer in Turlock before being hired by the Newman force in 2011.
English was Singh's third language and he had a thick accent but took speech classes to improve his communication, the police chief said.
RELATED: Pres. Trump weighs in after officials report suspect in officer's death was in country illegally
His death comes amid a political fight over immigration, with President Donald Trump and Congress at an impasse over funding for a border wall that has forced a partial government shutdown.
Trump tweeted about Singh's killing Thursday, saying it's "time to get tough on Border Security." He ended the post with: "Build the Wall!"
There is right now a full scale manhunt going on in California for an illegal immigrant accused of shooting and killing a police officer during a traffic stop. Time to get tough on Border Security. Build the Wall!— Donald J. Trump (@realDonaldTrump) December 27, 2018
Authorities were looking for a man seen in surveillance photos at a convenience store shortly before Singh was killed. Officials pleaded for help from the public and said they were following up on several leads.
#DEVELOPING #Newman Police Chief Randy Richardson referred to this photo of Cpl. Singh and his family, taken hours before he was shot and killed. “He wore that uniform because he loved being a police officer.” Singh’s dog Sam will be retired and stay with Singh’s family. #abc7now pic.twitter.com/BiVmMcexUu— Laura Anthony (@LauraAnthony7) December 27, 2018
"The sheriff's office will spare no expense in hunting down this criminal," Christianson said.
Singh pulled over the attacker as part of a drunken driving investigation and fired back to try to defend himself, Christianson said.
He was shot a few minutes after radioing that he was pulling over a gray pickup truck that had no license plate in Newman, a town of about 10,000 people, officials said. Singh died at a hospital.
A ground and air search began for the heavyset man pictured at the store with short, dark hair and wearing a silver chain, jeans, dark T-shirt and a dark jacket with white Ecko brand patches on the shoulders.
A truck believed to have been the one stopped by Singh was later found in a garage in a mobile home park about 4 miles (6 kilometers) from the shooting, where law enforcement officers were serving a search warrant, The Modesto Bee reported. Investigators were examining the vehicle, police said.
Richardson said his department of 12 is grieving Singh. Other agencies are lending a hand.
"He was living the American dream," said Stanislaus County Sheriff's Deputy Royjinder Singh, who is not related to the slain officer but knew him. "He loved camping, loved hunting, loved fishing, loved his family."
Ronil Singh was never in a bad mood and always had a smile on his face, Richardson said.
VIDEO: Witness records active scene after Police Corporal is shot in Newman
On his Facebook page, Singh posted pictures on Christmas Eve from a deep-sea fishing trip that produced a big haul of crabs and fish. His profile picture shows him smiling as he stands at a patrol car with his police dog - the same photograph of the officer released by the sheriff's department.
Singh is survived by his wife, Anamika, and their 5-month-old son.
"Please help us find this coward," Richardson said of Singh's killer. "We need closure, his family needs closure."
A memorial fund has been set up for Corporal Singh. Donations can be made in several different ways:
Any West America Bank Branch to the Corporal Ronil Singh Memorial Fund (Newman Police Department)
Stanislaus Sworn Deputies Association
Corporal Ronil Singh Memorial Fund
PO Box 2314
Ceres, CA 95307
The Stanislaus Sworn Deputies Association says please be aware there are NO other approved or authorized memorial fund or online fundraising accounts that will be established.