San Francisco prosecutors have declined to file charges in a stabbing case stemming from the Giants-Dodgers rivalry that resulted in the death of a 24-year-old Dodgers fan near AT&T Park last year.

District Attorney George Gascón said Friday that his office could not prove beyond a reasonable doubt that Michael Montgomery, 21, of Lodi did not act in self-defense when he stabbed Fort Bragg resident Jonathan Denver in a brawl on Sept. 25.

Authorities say subsequent interviews with witnesses and fight participants conducted over the past six months revealed that Denver and his brother had teamed up against Montgomery before he used a knife to protect himself.

Denver had left a Giants-Dodgers game that night with his father and brother in the eighth inning to go to a bar.

They encountered Montgomery and his friends in front of a liquor store near Third and Harrison streets in the South of Market neighborhood about 11:30 p.m. One of Montgomery's friends was wearing a Giants cap.

The two groups argued about the Giants-Dodgers rivalry, and the squabbling escalated into a brawl. Authorities say both groups were intoxicated: Denver's group on alcohol and Montgomery's group on marijuana.

Montgomery was booked into county jail on suspicion of homicide, but he was released two days later when prosecutors concluded there was not enough evidence to file charges.

Six months later, prosecutors said the circumstances have changed but charges will still not be filed. Gascón explained that in California, prosecutors not only have to prove their cases beyond a reasonable doubt, but they also have to prove the defendant did not act in "lawful self-defense."

Evidence from a district attorney's office investigation determined prosecutors would not be able to prove that.

The two brothers collectively weighed about 150 pounds more than Montgomery and they double-teamed him, Gascón said. Witnesses told investigators that Montgomery had a bottle in his hand for self-defense when Denver's brother grabbed an aluminum chair, featuring a Dodgers logo, and hit Montgomery on the head with it.

Montgomery dropped the bottle and stabbed Denver, who had been punching him during the assault, witnesses said.

"With multiple sources indicating how the event transpired, it makes it impossible for us to meet our burden and prove beyond a reasonable doubt that Mr. Montgomery was not acting in self-defense," Gascón said in a statement. "We are ethically obligated to decline to prosecute this case."

Prosecutors came to the conclusion after investigators reinterviewed "all percipient witnesses, including members of Mr. Montgomery's group, Mr. Denver's group and independent witnesses," Gascón said. Especially key to the investigation was the reinterview of Denver's brother.

"Our hearts go out to the victim's family," Gascón said. "The loss of a loved one in this manner is indeed tragic."

Jill Haro, Denver's aunt, said Friday that she was disappointed in the decision and had been reaching out to the district attorney's office for four weeks to no avail.

The past six months have been difficult for the family, especially for Denver's father and brother, who watched him die, Haro said.

"I'm disappointed," she said. "I think right now we just have to talk as a family and see where we go from here."