A homeless man was attacked and set on fire between downtown San Diego apartments Wednesday, the fourth attack on transients in as many days by the same assailant who has killed two men and left two, including the latest, in critical condition, police said.

Police officials said Wednesday that the person of interest seen in images shared with the public is now a suspect. Homicide Capt. David Nisleit said investigators are sure the man committed all four crimes, although he did not detail why they are certain. He implored the public to help find the man, adding that the department has already received many leads, and plans to follow up on every one.

"We really need the public's assistance," Nisleit said. "I truly believe that's how we're going to make this case. Someone's gonna recognize this person and give us who it is, and we're going to find him and arrest him."

Mayor Kevin Faulconer and Chief Shelley Zimmerman said cracking the case is the department's highest priority. Officers are informing those who are homeless of the attacks, showing them photos of the suspect, working to connect people with services and shelter, and advising them to be particularly aware of their surroundings.

"These evil acts of violence are some of the worst I've seen in my 34 years in law enforcement," Zimmerman said.

View the photo gallery: HOT team canvasses homeless after assaults

Nisleit stopped short of calling the assailant a serial killer, which is defined as a person who commits three or more related homicides, but described the attacks as a series.

“We consider him extremely dangerous,” Nisleit said. “We need to have him removed from the community as soon as possible.”

The spree began Sunday. Police said the victims all suffered similar and significant trauma to the upper torso, but would not detail the injuries. The last three attacked had been sleeping; all had been alone. Police don't believe the victims are connected in any way.

Angelo De Nardo was the first victim. He was found ablaze under the Interstate 5 bridge at Clairemont Drive in Bay Park about 8 a.m. An autopsy concluded the 53-year-old had suffered extensive injuries before being lit on fire, homicide investigators said.

Police said a man was seen carrying a gas can while running across northbound freeway lanes about two blocks away. He was described as between 30 and 50 years old, wearing a tan or brown jacket or sweatshirt and a baseball cap, and was carrying a backpack.

San Diego police have identified this man captured by surveillance cameras as the suspect in a series of attacks on homeless men that have left three dead, one critically hurt. San Diego Police Department — San Diego Police Department

Can you identify this man?

Anyone with information about this man or the attacks is asked to call police at (619) 531-2293 or Crime Stoppers anonymously at (888) 580-8477.

The victims of Monday's attacks were identified Wednesday. Manuel Mason, 61, was found critically injured in the Midway District about 4:50 a.m. He is expected to survive. Shawn Longley, 41, was found dead near the tennis courts at Robb Athletic Field in Ocean Beach about 6:10 a.m.

Witnesses to Wednesday's attack heard a commotion and saw one man crouched over another behind Koll Center Apartments south of Broadway at Columbia Street. One man ran to help the victim and yanked a flaming towel off of him, saving him from burns, Nisleit said.

He said the towel had been soaked in a flammable liquid and placed on the 23-year-old victim. He said the man suffered “significant trauma” to his upper torso, but he declined to describe what kind of weapon may have been used.

The victim's family had not yet been notified of his condition, so his name was not released.

The assailant was described only as a man of medium height and weight, wearing a hooded sweat shirt with the hood drawn tightly around his face, and possibly in jeans.

A resident of the nearby Park Row condominiums, who did not want his name published out of concerns for his safety, said he witnessed part of the Wednesday attack after a banging noise woke him.

"It sounded like a stick hitting a wall," he said, rapping forcefully on a wooden table with his knuckle to illustrate the sound. "Kind of like that - four or five bangs."

ATTACKS ON HOMELESS

He looked outside toward the east and saw what he thought was a blanket on the ground. Then he watched as a man walked south toward the blanket, sprayed it from what might have been an aerosol can, and lit the blanket on fire. The man then ran north, he said.

He was about to call 911 when another man ran up to the blanket and pulled it away, revealing a man lying beneath it. That's when the witness first realized there was a victim.

"He didn't try and stop the man from putting him on fire, so he must have been unconscious," the witness said, adding that the victim moaned as the blanket was pulled away.

"I can't get it out of my mind - seeing that guy light that other guy on fire. I haven't been able to stop thinking about it," he said. He called the assailant's movements "quick" and "intentional."

He added that he was focused on the fire, but believes the attacker was wearing a light-colored or white sweatshirt.

Nisleit said investigators on Wednesday would be checking for security camera video and “talking to a ton of witnesses.”

What you can do Homicide detectives have asked anyone with information about the attacks to call police at (619) 531-2293 or Crime Stoppers anonymously at (888) 580-8477.

Councilmember Todd Gloria, who also is chair of a council group that works to address homelessness, said the recent attacks serve as a reminder that more needs to be done to get those living on the streets into more permanent housing.

“It shouldn’t take these assaults and murders to make that point,” he said.

He added that on a regular basis there are beds in shelters that are unclaimed, for a variety of reasons including, for example, restrictions on pets. He said he hoped more transients would take up beds that are available instead of choosing to sleep on the streets.

The San Diego Police Department has two homicide teams, several patrol divisions, and its Homeless Outreach Team on the investigation, which includes alerting homeless people to not sleep or walk alone. A photo of the suspect is being circulated as widely as possible in hopes someone will recognize him.

View the Video Suspect in attacks on homeless

“It’s extremely unsettling for the community, for the homeless and for the Police Department,” Nisleit said. “That’s why we’re ‘all-in.’”

Several community organizations that run shelters or otherwise advocate on behalf of the homeless said they are working with police to spread the word about the assault series.

“I've never seen anything like this,” said Bob McElroy, president of the Alpha Project, which offers services to the homeless. “It’s scary, bizarre, freaky.”

He said the group’s outreach teams have been talking to people on the street this week but will expand the effort to encampments outside the city limits. Their message: stay vigilant.

“We’re telling everyone to cluster together until they catch him,” McElroy said Wednesday. “Sleep in groups, stay in lighted areas. Have someone watch over the group if you can.”

He said the organization received a donation several months back and they’ll be using some of that funding to try and persuade people, especially those who are infirm or who don’t like to sleep in groups, to stay at a hotel.

“There are some folks out here who are resistant to traditional housing, but they might go in if they’re scared. We’ll be able to put them in a hotel,” McElroy said. “We’re going to try and convince them in by saying, ‘Look, let us put you up for a few days until they catch this bastard.’ ”

Michael McConnell, who also works with the homeless, said on Wednesday that more people he’s spoken with are aware of the attacks, and they’re afraid and angry. But plenty of others he spoke with don’t know, or don’t care.

"I think there are a lot of people that are so beaten down that they just don't care,” McConnell said. “They're existing but they don't care. It's really sad."

Some homeless men said Wednesday they felt that sleeping among a group of strangers can be as dangerous, in its own way, as sleeping alone.

James Johnson said he sleeps by himself, but he’s close to the Broadway police headquarters and doesn’t believe the serial attacker would strike so close to law enforcement.

“I’m not a social person,” he said. “I’m more of a loner. I always try and find a good spot.”

A number of transients said they’ve armed themselves in the wake of the attacks. Some pulled out knives, one man brandished a pink bat.

“If someone comes up and I don’t know them at night time, with people going around beating up the homeless and taking them out at night time, I’ll tell them to back away, and come out with this (bat),” said John Henry.

“People are being more careful, but we’re also trying to go about our business,” said Henry's friend, Jessie Braisher.