The Los Angeles police are searching for a man they believe shot three homeless people dead this week, prompting officials on Friday to urge anyone sleeping on the streets to find shelter indoors or avoid sleeping alone until the killer is found.

The three victims — all men who were in alleys or open areas by themselves — were shot and killed in the early morning hours, said Chief Michel Moore of the Los Angeles Police Department. The killings took place in different locations across central Los Angeles on Sunday, Monday and Wednesday, but he said that investigators had discovered similarities among the three killings, based on physical evidence and camera footage, that led them to believe the events were connected.

“This is a killer who is preying on the unhoused,” Mayor Karen Bass said at a news conference on Friday afternoon. “Many friends and family members know how to reach their unhoused relatives. We need you to contact them today. We need you to tell them about this danger.”

The authorities asked the public to help them to identify a dark-colored sedan and a person appearing in a surveillance image, whom they described only as a male. Federal and county investigators are also working on the case and trying to determine whether there have been more victims.

Mr. Moore told reporters that he was particularly concerned about homeless people who might be isolated away from larger encampments, which often function as small communities.

At the news conference, the mayor said that emergency shelters that typically open during severe weather events would be made available for homeless residents.

Dozens of leaders at shelters in Los Angeles were called to an emergency meeting on Friday afternoon.

“The Police Department has obviously seen something that disturbs them and wants to get the word out — they’re being very proactive,” said David Prentice, the C.E.O. and president of the Midnight Mission, a longtime shelter in downtown Los Angeles. “Our outreach teams are having conversations with people who tend to isolate and be solo.”

Mr. Prentice said that it was not a moment of panic but more an elevated security situation. The organization already has a policy of taking in anyone who needs a place to sleep. Those who are resistant to shelters can stay in the outdoor courtyard, where security has been increased.

At the edge of Skid Row on Friday afternoon, pedestrians walked down 7th Street, unaware that one of the men had been killed nearby. The area, where warehouses can take up an entire block, is known for its homeless encampments and residential hotels with low-income clientele. During rush hour, cars tend to barrel down the trash-lined streets where sidewalks are dotted with portable toilets and shopping carts.

Anthony Taylor, 57, knows the neighborhood well and lives in transitional housing nearby. He said he was homeless off and on for three decades and spends much of his time hanging out on the block. Mr. Taylor said he often encounters people struggling with a mental illness or a drug addiction like the one he battled.

He wasn’t sure news of the homicides would trouble many.

“I don’t think anybody will feel one way or another because they already see so much violence,” Mr. Taylor said. “They’re living day to day and going through so much transition in their own life. It’s rough to be out here.”

Although the moment will certainly strain resources, Dr. Va Lecia Adams Kellum, the chief executive of the homeless-services agency for the city and the county of Los Angeles, said that she hopes that homeless people who find shelter in the days ahead stay indoors.

“This tragedy of loss is also a time that we’re going to talk to people and remind them how deadly the streets are, how vulnerable they are,” she said, “and how much we want people to not return.”

The last count, released in June, found that 46,260 people were homeless in the city of Los Angeles — a number that the mayor was elected last year with an urgent mandate to decrease.

Encampments proliferated during the pandemic and had become a persistent source of tension; many Angelenos said they were fed up with feeling unsafe and regularly witnessing horrific scenes of human suffering as they passed by the clusters of tents that lined sidewalks in neighborhoods ranging from Skid Row in the industrial downtown core, to the street medians of Brentwood, a ritzy enclave that is home to celebrities.

The killings this week served as a stark reminder that homeless residents are often the most profoundly vulnerable, enduring harsh conditions on the streets, night after night, in one of the nation’s most intractable humanitarian crises.

“The public is often very frightened of people who experience homelessness,” said Dr. Margot Kushel, director of the Benioff Homelessness and Housing Initiative at the University of California, San Francisco. “But what they don’t realize is that people experiencing homelessness are frightened all the time.”

Those experiencing homelessness “don’t have a door to lock,” she said. They are exposed, not just to the elements, but to shocking levels of physical violence.

Dr. Kushel said that in a recent statewide survey — widely considered a definitive report on homelessness in California — her team had found that more than a third of people who were homeless had been sexually or physically assaulted since they were last housed. She said that homeless people were more likely to be the victims of violence perpetrated by a stranger, rather than someone known to them.

In 2022, 92 people who were homeless or “transient” were killed in Los Angeles, a number that has steadily risen in recent years, according to Crosstown, a nonprofit newsroom that analyzes Los Angeles Police Department data.

It’s not just in the nation’s second largest metropolis that people sleeping on the streets have been targeted.

Last year, Stockton, Calif., about 80 miles east of San Francisco, was unnerved by the killings of seven people, some of them homeless, in the city and a nearby county. The local authorities said it was the work of a serial killer. A suspect in the killings, Wesley Brownlee, has since been charged with seven counts of murder.

This spring, the college town of Davis, Calif., was rattled by three stabbings that left two people dead, including a local pacifist who slept on a park bench. One woman, who was living in a homeless encampment, was injured.