SAN JOSE — Just days before his birthday, a 14-year-old boy was charged as an adult in the stabbing death of an Oakland man during a predawn gang attack in the heart of downtown San Jose in late July, authorities said.

Marvin Garcia was charged with murder and a gang enhancement in the July 29 killing of 33-year-old Michael Maynard near Market Street and Park Avenue, near Plaza de César Chávez park and the Tech Museum of Innovation.

He becomes one of the youngest persons charged as an adult in a Santa Clara County homicide. Two adult suspects, 21-year-old Luis Daniel Garcia — Marvin’s brother — and 20-year-old Luis Alvarez were charged with the same counts. All three are San Jose residents and appeared in court Thursday.

The fatal stabbing was preceded by a heated conversation over an undisclosed issue, authorities said. According to court documents, one witness told police that around 2:15 a.m., the three defendants, and possibly other men who were not arrested, chased Maynard across the park. They caught up to him, knocked him to the ground, and started to punch and kick him.

A second witness reported that the Garcia brothers yelled out gang slogans in the throes of the beating and that they slashed him while Alvarez kept inflicting blows.

After the attack, the assailants left Maynard on the ground, ran westbound across Market Street and got into a waiting car, authorities said.

Besides the two witnesses, the attack was captured at least partly on surveillance cameras, which were plentiful in the area. Both Luis Garcia and Alvarez are believed to be documented gang members, authorities said.

The two adult defendants were arrested the next day, and Marvin Garcia was arrested Saturday. When Alvarez was arrested, investigators noticed a reddish tint on his shoelaces, which later tested positive for blood.

In contrast to initial police statements doubting gang motives in the killing, the gang enhancements and allegations outlined in court documents suggest otherwise.

Sources familiar with the investigation said Maynard was believed to be a gang member in Alameda County, and that though the case involves gang elements, gang rivalry was not likely what spurred the attack.

Marvin Garcia, who will turn 15 later this month, is among the youngest homicide defendants in the county’s history, but his charges are not without precedent. In 2004, James Ortega became the youngest person ever charged an adult in a homicide case in Santa Clara County. He turned 14 — the minimum age for being tried as an adult — just 12 days before he gunned down two men during a gang brawl inside a fast food restaurant.

Three years later, Ortega became the youngest person in county history to be sentenced to life in prison when, at age 17, he was sentenced to 36 years to life.

Contact Robert Salonga at 408-920-5002. Follow him at Twitter.com/robertsalonga.