A California clerk was fatally shot during a robbery. The suspect won't be charged with murder.

A robber fatally shot a Northern California gas station clerk but won't be charged with murder because the victim wrongly chased and opened fire on the gunman, prosecutors said Thursday.

Ronald Jackson Jr., 20, was initially booked on suspicion of armed robbery and homicide in connection to the slaying of James Williams, 36, early Saturday at a Chevron station in Antioch, police said Wednesday.

Jackson and another man, who was still being sought Thursday, were fleeing the robbery when Williams chased and opened fire on them, police said. Jackson was struck in the leg and returned fire, killing Williams, police and prosecutors said.

Antioch detectives submitted their findings to the Contra Costa County district attorney's office, and prosecutors "elected to charge Jackson with robbery, possession of stolen property, and a firearm enhancement, but declined to charge him with murder," police said in a statement.

District Attorney Diana Becton remained silent on the matter until early Thursday evening when her office issued a lengthy statement, saying the decision to not file murder charges was based on a "legal and factual determination."

Becton said deadly force is legally justified as an act of self-defense when life or property is threatened, but not to go after someone to retrieve property once the threat of harm has subsided.

"In the eyes of the law, Mr. Williams' actions ceased to be self-defense when Mr. Williams pursued Mr. Jackson and the other suspect with a firearm — and continued to pursue Mr. Jackson after he shot him," she wrote.

Jackson has been charged with second-degree robbery, a special allegation of using a firearm in that crime and receiving stolen property worth more than $950 — crimes that could land him behind bars for as long as 15 years — but not homicide.

The decision stunned girlfriend and co-worker Annette Matamoroz, who was at the store when Williams was fatally shot.

"I totally disagree with this," a tearful Matamoroz told NBC News on Thursday. “It is murder. It doesn’t seem like murder; it is murder. What else could it be?"

Surveillance video showed Jackson and his accomplice running away from the Chevron and about 100 feet away from Williams when he shot Jackson in the back of his leg, Chief Assistant DA Simon O'Connell told reporters on Friday.

"He strikes one of those individuals who goes down to the ground and continues to then close the approximate distance between himself and the two perpetrators of that robbery," O'Connell said. "Further gunfire is made by Mr. Williams toward hose individuals."

Because Jackson was wounded outside the Chevron, a legal expert says the suspect could have argued he was leaving the scene and had a legal right to protect himself.

“Self-defense is a temporal concept. So in other words, had this event happened within the store, that would have been one thing," said Steven Clark, a San Jose criminal defense attorney and NBC Bay Area legal analyst.

"But when the guy ran away, the concept of the clerk being reasonably afraid, that changed considerably. Now you have a serious felony, what the robber did, but you don’t get to execute the guy under those circumstances. So the robber then obtained the right to self-defense.”

A pair of Contra Costa County prosecutors, meeting with reporters on Friday, stood firm on the office's decision. They even hinted Williams might have been in some legal jeopardy by opening fire outside the Chevron as the suspects fled.

The sprinting robbers had “achieved a clear distance” when Williams fired nine shots, one that brought down Jackson, Deputy DA Derek Butts said.

“I don’t want to come to a conclusion about that, but it’s crossed my mind,  hat analysis of how difficult it would be, the case would be, where the clerk is now potentially being prosecuted for using force,”  Butts said.

“We’ve encountered this situation where someone’s use of force exceeds what is reasonable as self-defense. And in this case, because Mr. Williams' use of force was unreasonable ... perhaps it would be that he was liable for some crime depending on the outcome of the shooting."

The victim's girlfriend Matamoroz insisted Williams isn't at fault and said he'd still be alive today "had they not come to the store."

"They came with those intentions" — that is, to commit a crime, she said. "They didn't come here just to go shopping."

While he agreed with Becton's decision not to seek a murder prosecution, Clark faulted her for failing to quickly and forcefully explain the rationale to an understandably concerned and confused public.

Becton didn't issue her statement until about 5 p.m. PST Thursday before sending two of her top deputies to meet with reporters at noon Friday to explain the charging decision.

"There's a general doctrine that when you are the initiator of the problem, like the robber was, your self-defense rights are lessened than if you are the victim, like the clerk," Clark said Thursday.

"There's going to be potential for criticism here. 'Why are we bending over backward to not charge an armed robber with the murder of an innocent store clerk who is protecting his premises?' That seems to stand the law on its head. That seems to stand logic on its head. The DA needs to carefully articulate to the public why this decision was made."

Williams' death will play a key role in seeking maximum punishment, according to Becton, even though Jackson won't be prosecuted for the clerk's slaying.

Becton wrote that, "the reality of Mr. Williams' tragic death will be a substantial factor in the DA's position in asking for greater penalties in the sentencing of the defendants."

Matamoroz worked at the same Chevron but wasn't on the clock when she was keeping Williams company before his death.

Matamoroz was in the bathroom when Williams told her to stay put.

“He had told me: 'They’re trying to rob this place. Just say here. Just stay here in the bathroom,'" she said.

"He was a loving, caring, genuine man. You don’t come across too many people like that in life. He actually genuinely cared. He cared about people he didn’t even know. He wanted to help everybody. He wanted everybody to be all right all the time. If he could make it better, that's what he would do."

Matamoroz then broke down in tears, and her voice trailed off: "That’s why I want him here, because he would be making this better and I wouldn’t be feeling like this.”

This article was originally published on NBCNews.com