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San Diego Union-Tribune via AP, John Gibbins
San Diego's Metro Arson Strike Team and SDPD homicide team gather evidence from the sidewalk and grassy area where a homeless person was attacked in downtown San Diego early Wednesday morning, July 6, 2016. It is the fourth attack on homeless men in San Diego in four days, two of which resulted in death.

SAN DIEGO — A series of attacks on San Diego's homeless population this week have left two transients dead and a community on edge as police search for one man suspected of the bloodshed.

Advocates are warning those who live on the streets about the assailant who separately attacked four men as they slept in the early morning hours. The violence occurred in different parts of the city over four days, and at least one victim was set on fire.

"There's no doubt our city has been shaken by these gruesome attacks," Mayor Kevin Faulconer said at a news conference. "The last few days have been particularly harrowing and emotional for those who struggle with homelessness. These crimes are being committed against some of our city's most vulnerable people."

The Alpha Project, which provides temporary housing in San Diego, has sent eight representatives to canyons, freeway ramps and other encampments to encourage transients to sleep in groups and in lighted areas, said Bob McElroy, the group's president.

"People are freaking out," said McElroy, who has been distributing pepper spray.

Capt. David Nisleit declined to speak in detail about why police believe the attacks are connected but mentioned the way they were carried out and witness interviews. Police had no information linking the victims.

"It's really hard to say what our suspect's motive is," Nisleit said of the man seen on convenience store surveillance video wearing a cap, jacket and backpack. "All I can say is these are senseless attacks."

In the latest violence, a 23-year-old man was seriously injured Wednesday and not expected to survive injuries to his upper body. A news release said he was set on fire, though Nisleit previously said a witness pulled away a burning cloth that the attacker put on the victim before fleeing. The man's identity has not been released.

Police began investigating the attacks when the badly burned remains of Angelo De Nardo, 53, were found Sunday between Interstate 5 and some train tracks.

He suffered extensive trauma to his upper torso and died before his body was set on fire. Police have not explained the nature of the injuries or what kind of weapon was used.

The next day, officers responding to a 911 call found Manuel Mason, 61, who suffered life-threatening injuries to his upper torso and remained in critical condition Wednesday. A few hours later, police discovered the body of Shawn Longley, 41, who had bled from the upper torso.

The string of violence comes four years after rampage in nearby Orange County left six people dead, including four homeless men as well as a woman and her son. Itzcoatl Ocampo, 25, was charged in 2012 but died in jail before trial. Several of the victims were stabbed.