By Alex Dobuzinskis

(Reuters) - A homeless man in San Diego was set on fire and critically injured early on Wednesday in the fourth in a series of attacks on transients in the city that have killed two people and are believed to be the work of a single suspect, police said.

San Diego police Captain David Nisleit told a news conference that witnesses to the latest attack saw an individual crouched over the flaming victim before running away.

"We need to locate this subject who is responsible now for four attacks, leaving two people dead and two people critically injured," Nisleit told reporters.

The latest victim was injured in his chest, in addition to being burned, Nisleit told local media.

San Diego television station 10 News, an ABC affiliate, said the attack occurred outside a downtown apartment complex.

The station aired video taken soon after the assault showing fabric burning on a sidewalk and a person lying on the grass nearby as someone leaned over the victim, apparently offering comfort.

The attacks on homeless people in California's second most populous city began on Sunday when the body of a homeless man was discovered on fire between a highway and train tracks in the Mission Bay area of San Diego, police said.

The victim has been identified as 53-year-old Angelo De Nardo and an autopsy determined he had been killed before being set ablaze, according to City News Service.

On Monday before dawn, a 61-year-old man was discovered bleeding, with trauma to his upper body, less than 4 miles south of the first attack, police said.

He was taken to a hospital with life-threatening injuries, police said.

On Monday morning, just over an hour after the discovery of the badly wounded man, a third victim was discovered near tennis courts in the Ocean Beach neighborhood, police said.

He had suffered trauma to his upper torso and was dead when police arrived, police said.

Police said on Monday they were seeking a "person of interest" in connection with the attacks.

Witnesses saw the person of interest near where the first attack occurred on Sunday and he was seen on surveillance video inside a store wearing a backpack, police said.

(Reporting by Alex Dobuzinskis in Los Angeles; Editing by Peter Cooney)