NEW YORK (AP) — A gunman who killed three Iranian indie rock musicians and injured a fourth person inside a Brooklyn apartment on Monday before killing himself was upset because he had been kicked out of another band last year, police said.

Ali Akbar Mahammadi Rafie killed himself on the roof after struggling with a member of his former band, the Free Keys, police said. Investigators believe a guitar case found on an adjoining roof may have been used to carry the assault rifle used in the attack.

Rafie, 29, “was upset that he wasn’t in the band anymore,” said New York Police Department spokesman John McCarthy. Investigators suspect the shooter and his former Free Keys band mates may have had an argument over money, he added.

Two of Rafie’s victims were brothers and members of the Yellow Dogs, a band that came to the US from Iran three years ago after appearing in a film about the underground music scene there, according to band manager Ali Salehezadeh. The third person killed was a musician but not in the Yellow Dogs band, Salehezadeh said. The person injured was an artist, he said.

It wasn’t immediately clear why Rafie opened fire on members of another band, although musicians in both groups knew each other and some lived in the same building, Salehezadeh said.

Rafie knew his victims but he hadn’t spoken to them in months because of a “very petty conflict,” Salehezadeh said, declining to give specifics.

“There was a decision not to be around each other,” he said. “They were never that close to begin with. … This was nothing. We thought it was all behind us.”

The four victims lived in a row house in East Williamsburg, an industrial neighbourhood home to mostly warehouses where artists can rent cheaper space than in trendier parts of the city.

The rampage erupted shortly after midnight when the gunman climbed down from the roof to a third-floor terrace and opened fire through a window, killing 35-year-old Ali Eskandarian.

The shooter then killed brothers Arash Farazmand, 28, in a third-floor bedroom and Soroush Farazmand, 27, in a second-floor bedroom while he was on a bed using his laptop computer, police said.

An unidentified tenant was hit in the arm before Rafie and his former band mate from Free Keys struggled over the gun until the clip fell out, police said. Rafie put the clip back in the rifle, went back to the roof and shot himself in the head.

The gun was found next to the body. Kelly said it had been purchased in upstate New York in 2006 and police were investigating its history.