Investigators have tied the killings to Itzcoatl Ocampo, 23, of Yorba Linda, who was detained Friday night after a fourth homeless man was found slain in the parking lot of a fast-food restaurant, Welter said.
Witnesses and bystanders at the crime scene chased Ocampo on foot, and he was captured by a police officer who was part of a perimeter set up in response to dozens of 911 calls and other reports.
Three other homeless men have been found stabbed to death in north Orange County since mid-December, and a task force had been looking for the single suspect they believed was responsible for all three.
"We are extremely confident that we have the man that is responsible for the murders of all four homeless men in Orange County," Welter said. "We plan to request from the district attorney that he be charged with four counts of murder."
Neither Welter nor any of the other police chiefs, FBI agents or political officials who spoke at the news conference would give any information on the evidence against Ocampo, or any suspected motive.
Ocampo is being held without bail at the Anaheim jail, and will be transferred to Orange County Jail and appear in court next week, Welter said.
Ocampo could not immediately be reached for comment. A phone number listed in his name rang without an answer, and no one answered the door at two addresses listed in his name.
Also Saturday, mourners wept at the scene of death of the latest stabbing victim, who was described by friends as a Vietnam War veteran in his 60s named John. They left flowers and signs, one of which read "We love you, John."
The victim was found between 8 p.m. and 9 p.m. Friday in the parking lot of a Carl's Jr. that is surrounded by banks and other businesses at the wide, busy intersection of La Palma Avenue and Imperial Highway in Anaheim, police said.
A candlelight vigil for the man and the other victims was planned for Saturday evening.
Marilyn Holland, an Anaheim resident who befriended the victim and regularly brought him oatmeal raisin cookies, said he was uncharacteristically nervous since police warned him to stay vigilant in the days after the killings began.
"He told me he thought he was being followed," Holland said. "I told him after pay day I was going to get him a cellphone, so he could call 911 if anything happened. Normally he would refuse help but he was willing to accept the phone because he was scared." Holland was paid Friday but didn't get the chance to get the phone to her friend.
Several witnesses reported an assault in progress, and officers arrived to find the homeless man dead near a trash bin in the restaurant parking lot. Witnesses followed a man who ran from the lot and led police to him, Anaheim police Sgt. Bob Dunn said.
"We were having dinner in the area and saw about 40 police cars scream into the parking lot. I ran over and hugged my friend, screaming, `Please tell me it's not John!' But it was," Holland said, fighting back tears.
Police set up a large containment area at the crime scene in a search for the killer and scoured nearby neighborhoods, including a mobile home park, Dunn said.
A police bloodhound traced the scent from Ocampo's belongings back to the scene where the attack occurred, about 10 miles northeast of the Disneyland Resort, authorities said.
Virtually everyone who spoke at the Saturday news conference thanked what Placentia police Chief R.A. Hicks called the "brave citizens" who chased the suspect and made sure he was arrested.
A task force of law enforcement officers from Anaheim, Placentia, Brea, the Orange County Sheriff's Department and the FBI had been formed to investigate the killings of the three other homeless men.
James Patrick McGillivray, 53, was killed near a shopping center in Placentia on Dec. 20; Lloyd Middaugh, 42, was found near a riverbed trail in Anaheim on Dec. 28; and Paulus Smit, 57, was killed outside a Yorba Linda library on Dec. 30.
It was Smit's killing and its similarities that led police to strongly believe they were seeking a serial killer of homeless men and prompted the creation of the task force, said Brea police Chief Jack Conklin.
Police and advocates had been urging those living on the streets to head inside or buddy up in the wake of the killings.