ANAHEIM (AP) — Danny Hopkins was making a quick stop at a drug store to buy his wife a pack of cigarettes when a man burst inside and screamed for help.

Hopkins ran from the store and spotted someone being stabbed to death in the parking lot of a nearby fast-food restaurant.

"I'm yelling as loud as I can, 'hey — stop!' at the top of my lungs. He just kept going and kept going," Hopkins told The Associated Press Wednesday about the Jan. 13 attack.

Fumbling to call 911 on his cell phone, Hopkins chased the suspect across the Anaheim strip mall and into a mobile home park, where the 32-year-old forklift drive lives with his his wife, two children and mother,.

Authorities surrounded the area and arrested Itzcoatl Ocampo, a 23-year-old former Marine from nearby Yorba Linda.

Prosecutors have charged the Iraq war veteran with the murders of four homeless men in a nearly month-long spree that prompted police to fan out across suburban Orange County to urge the homeless to seek shelter indoors.

The Association of Orange County Deputy Sheriffs offered a $5,000 reward for information leading to the capture of the killer in a county that's more well-known as the home to Disneyland and multi-million dollar beachfront homes than for its thousands of homeless people.

The association handed the reward check to Hopkins at a news conference Wednesday outside the fast-food restaurant where a memorial of candles, flowers, teddy bears and signs has been created in memory of 64-year-old victim John Berry.

"While we never encourage citizens to put themselves in danger, his actions saved unknown lives," said Tom Dominguez, the association's president.

Hopkins said he had previously given money to Berry, the man he saw being stabbed.

"I did what I hope anybody would do if you see somebody in trouble," he said. "That's just your first reaction — to help them."

Hopkins intends to use the reward to get caught up on unpaid bills and help his mother, who lost her job a few weeks ago.

He said he doesn't feel like a hero because he saw Berry die.

"I'm just a guy who did the right thing. John was a Vietnam vet — he's a hero. That's a real hero," Hopkins said at the news conference.

Prosecutors said Ocampo stalked each of his victims and stabbed them repeatedly with a knife that could cut through bone. Ocampo was caught with blood on his hands and face, prosecutors said.

Ocampo's father, Refugio Ocampo, who also is homeless, said his son was troubled after he returned from Iraq in 2008.

Itzcoatl Ocampo was being held without bail and is scheduled to be arraigned Feb. 17.

If convicted, he could face a minimum sentence of life without parole.

Prosecutors said Ocampo targeted Berry after he appeared in a Los Angeles Times story about the serial killings.

In addition, James Patrick McGillivray, 53, was killed near a shopping center in Placentia on Dec. 20, and Lloyd Middaugh, 42, was found near a riverbed trail in Anaheim on Dec. 28. Paulus Smit, 57, was stabbed to death outside a library in Yorba Linda on Dec. 30.