Updated at 2:45 p.m. July 7, 2016

An all-out search continued Thursday for an unidentified assailant responsible for a four-day spree of random violence that has killed two homeless men and critically injured two others in various San Diego neighborhoods.

Mayor Kevin Faulconer and police Chief Shelley Zimmerman have called the effort to identify and capture the assailant — who began attacking transients last weekend, setting two of them on fire — the primary law enforcement priority for the city.

During an update late Thursday morning, SDPD Capt. David Nisleit said the “very violent person” carrying out the brutal attacks may be using public transportation and urged bus and trolley riders to be on the lookout for him.

Surveillance cameras at a convenience store near where the first victim was found captured images of a man believed to be the perpetrator. In the video footage, the dark-haired suspect, who appears to be in his 20s or 30s, is carrying a backpack and wearing dark gloves, light-colored pants, a brown long-sleeve shirt or jacket over a dark undershirt and a green short-billed Greek fisherman-style cap.

About 8 a.m. Sunday, the assailant killed 53-year-old Angelo De Nardo of San Diego and set his body on fire in an open area off the 2700 block of Morena Boulevard in Bay Park, according to police. An autopsy determined that De Nardo died prior to being set ablaze.

Shortly before 5 a.m. on Independence Day, transient Manuel Mason, 61, was found bleeding from wounds to his upper body near Valley View Casino Center in the Midway district. He remains hospitalized in critical condition.

Less than 90 minutes after the assault on Mason was reported, 41-year-old drifter Shawn Longley was discovered dead at a park on Bacon Street in Ocean Beach.

About 5 a.m. Wednesday, a 23-year-old homeless man was gravely injured in the area of Broadway and State Street, and a towel he was sleeping under was doused with a flammable liquid and set ablaze, according to police. The victim, whose name has not been released, was hospitalized in extremely critical condition and had a poor prognosis for survival, officials said.

Authorities have declined to disclose the cause or causes of the targeted men’s non-fire-related injuries, though Nisleit said all four victims had suffered significant trauma to their upper bodies.

Over the last five days, police have been warning the homeless to protect themselves by remaining vigilant and aware of their surroundings at all times, avoiding sleeping alone on the streets and staying in open, well-lit areas.

Late Thursday morning, Father Joe’s Villages, a major local homeless-services provider, announced that it was seeking funding to provide additional beds at its downtown St. Vincent de Paul Village shelter as a means of helping protect those who have nowhere else to stay.

On Wednesday, the police chief told reporters that she had made bringing in the perpetrator of the attacks her officers’ primary objective as they go about the rest of their daily duties.

“These evil acts of violence are some of the worst that I have seen in my 34 years in law enforcement,” Zimmerman said. “This killer has targeted some of our city’s most vulnerable citizens while they are asleep.”

The mayor said police officers were fanning out to make those in the transient community aware of the attacks and to keep them safe.

“These crimes against some of our city’s most vulnerable people are absolutely reprehensible,” Faulconer said.

Anyone who spots a man believed to be the assailant was urged to avoid approaching him and immediately call 911. Anyone with information about the case was asked call the SDPD homicide unit at (619) 531-2293.

Anonymous tips can be submitted by calling Crime Stoppers at (888) 580-8477 or by contacting the agency via email at sdcrimestoppers.org.

— City News Service