Los Angeles officials warned Friday that three homeless men had been found shot to death on city streets within a span of days, and called on the public for help identifying a suspect.

“We believe a single individual approached each one, and shot and killed each one as they slept,” Michel Moore, chief of Los Angeles police, said at a press conference.

Moore declined to label the suspect a serial killer, but he noted how each attack had been made in the early hours of the morning on people who were alone. The police department is also looking to see if any similar crimes have gone unreported.

“I want to be very clear about what we are facing today: This is a killer preying on the unhoused,” Mayor Karen Bass said.

“Our message to the unhoused community is clear: Do not sleep alone tonight. Seek shelter, seek services, stay together, seek support, and we need your help to get the word out.”

Bass said she directed the city’s homelessness services to spread the message to unhoused people, and urged people with friends and loved ones experiencing homelessness to make sure they know about the increased risk.

“We are working to open emergency shelters that are open during difficult weather conditions,” the mayor said. “We want those open now, so we can get as many people who are unhoused in those shelters.”

A tip line has been set up, patrols increased and a task force established to help identify a suspect, Moore said.

Authorities released a blurry photo of a dark-colored vehicle that is believed to be related to the crimes, along with a photo of a possible suspect shown wearing a light-colored hoodie. It is not known whether other people may have been involved.

The incidents are believed to have begun Sunday at around 3 a.m. Pacific time, when a 37-year-old was killed. A second man, 62 years old, was killed on Monday at 4:55 a.m., and a third, 52 years old, was killed Wednesday at about 2:30 a.m.