EVERETT — A Tulalip-area man was convicted of first-degree murder Wednesday for opening fire at a riverside park in Arlington last year, ending the life of Ryan Mumm.

Dennis Watters, 42, claimed that he acted in self defense when he began shooting during a July 14, 2012, confrontation at Blue Stilly Park in Arlington.

One of the bullets struck Mumm in the temple as he sat in the passenger seat of a car. The 20-year-old died the next day.

Watters was charged with first-degree and second-degree murder. He also was charged with two counts of first-degree assault for later firing at the car that was carrying a fatally injured Mumm and another man as it sped away on Highway 530.

Jurors began deliberations Tuesday afternoon.

They found Watters guilty of the most serious charge, but could not reach a verdict on second-degree murder. Instead, they found him guilty of a lesser-included offense, first-degree manslaughter, which is recklessly causing a death without intent.

The verdict reflected the prosecution’s leading theory. Deputy prosecutor Cindy Larsen had argued that Watters committed first-degree murder because he fatally shot Mumm “under circumstance manifesting an extreme indifference to human life.”

In other words, jurors didn’t need to believe that the killing was intentional, but even so, the circumstances were unusually vicious. Larsen alleged Watters knew he was putting others in grave risk and ignored the danger.

Jurors also convicted Watters of both counts of assault. They found that he was armed with a gun while committing all the crimes.

The conviction means Watters likely will spend at least three decades in prison. He is scheduled to be sentenced Dec. 16.

The trial was plagued by problems with jurors. Superior Court Judge Michael Downes has scheduled a hearing Monday to decide whether to punish one man who opted not to show up after taking the oath to hear testimony in the case.

The defendant was led away in handcuffs Wednesday.

Mumm’s family looked on. The verdict brought them to tears. They later hugged the detectives and deputy prosecutor, thanking them for their work in the case.

The fatal dispute was sparked by somebody not paying for a small amount of marijuana.

Watters told detectives that he had gone to the park to back up a friend. He did not testify during the trial, but jurors heard the recorded statement he provided to Snohomish County sheriff’s detectives.

Watters told them it was unwise for him to have become involved, and that he never intended to hurt anyone.

Prosecutors alleged that Watters was determined to be a hero and killed Mumm in cold blood.

Writer Scott North contributed to this report. Diana Hefley: 425-339-3463; hefley@heraldnet.com.