In an alert, the California Highway Patrol said the suspect, whose name was not known, is considered to be armed and dangerous.
Newman Police Officer Ronil Singh, 33, had called in the traffic stop early Wednesday, saying he was pulling over a grey Dodge pickup truck that had no license plate, and a few minutes later reported shots fired over his radio, Stanislaus County Sheriff's Department Deputy Royjinder Singh said.
Multiple agencies responded and found Singh with gunshot wounds. They rushed the husband and father of an infant boy to a hospital, where he was pronounced dead.
Ground and air units were searching for the suspect based on surveillance photos taken at a convenience store shortly before the attack in Newman, about 100 miles (160 kilometers) southeast of San Francisco.
In the photos, the heavyset suspect with short dark hair is wearing a thick chain necklace, jeans, a dark hoodie and dark-colored T-shirt. Surveillance footage also captured the suspect's vehicle, a grey Dodge Ram pickup with paper dealership plates, matching the description that Singh had called in. Authorities later located the vehicle and said it was being processed for evidence.
"We have sent out information to law enforcement up and down the state, as well as outside California," said Sgt. Tom Letras, a sheriff's spokesman.
Singh was a native of Fiji and had worked since July 2011 for the police department in Newman, a town of about 10,000 people. Earlier in his career, Singh worked as a deputy with the Merced County Sheriff's Department.
"He was living the American dream," said Stanislaus deputy Singh, who is not related to the slain officer but knew him. After emigrating from Fiji, Ronil Singh went to the police academy and got his dream job, the deputy said, adding that "he loved camping, loved hunting, loved fishing, loved his family."
On his Facebook page, Ronil Singh posted pictures on Christmas Eve from a deep-sea fishing trip that produced a big haul of crabs and fish. The profile picture shows him smiling as he stands at a patrol car with a dog — the same photograph of the officer released by the sheriff's department.
Singh is survived by his wife, Anamika, and a 5-month-old son, authorities said.
Outgoing California Gov. Jerry Brown offered condolences to Singh's family and said flags at the Capitol would fly at half-staff in his honor.
"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.
Anyone with information regarding the identity of the suspect to contact Detective Michael Fisher at 209-525-7083 or dispatch at 209-552-2468.