After the carnage, police found a guitar case and decided that bassist Akbar Mohammadi Rafie must have used it to carry his assault rifle as he went from roof to roof along Maujer Street in Brooklyn just before midnight on Sunday.

At midblock, Rafie, 29, came to a three-story building that was home to several indie rock musicians he had first played with back in his native Iran. He had been with the Free Keys back then, and they had joined a band called The Yellow Dogs in a hidden chamber in a west Tehran basement soundproofed with Styrofoam. Their audience had been a few friends who had received whispered invitations along with a warning not to park their cars nearby. A friend’s dad kept watch for the official forces of repression by pretending to water a plant outside.

“We were super lucky we didn’t get caught,” Koory Mirz of the Yellow Dogs later recalled in a video.

Both the Yellow Dogs and the Free Keys were featured in No One Knows About Persian Cats, a documentary about the underground music scene in Iran that won a prize at the 2009 Cannes Film Festival. The Yellow Dogs managed to escape to America and apply for political asylum in 2010.

Rafie and the other Free Keys were apparently delayed because not all of them had met the Iranian military service requirement. They arrived in New York in 2011, having first gone through India and Turkey. Rafie is said to have had a falling out with the other Free Keys in 2012. He likely felt further aggrieved after his ex-bandmates acquired two new members who were introduced to them by The Yellow Dogs.

Rafie is said to have been desperate to get back with the Free Keys as his former band and The Yellow Dogs each played at various venues in Brooklyn. The Yellow Dogs were a particular favorite at Brooklyn Bowl, where they became known for always giving their all. A September 29 rooftop party atop the Yellow Dogs’ buildings in the epicenter of hipsterdom was literally a high point for the band, just as their onetime basement gigs had been literally a low.

Here was an under-the-stars manifestation of the pop culture that is America’s greatest power. It had helped knock down the Berlin Wall and it had swayed the younger generations of countries liable to become only more intransigent in the face of our armed might. Not for nothing do the mullahs ban our music.

And the Yellow Dogs, along with the Free Keys, were turning our music into their music with unique flair, making it a common denominator between the freer spirits of Tehran and those in New York. To listen to them was to hear hope electrified with a beat that invited you to dance with happy abandon.

“A punk/dance band from Tehran,” the Yellow Dogs called themselves on Twitter.

In a video, one of the band members said of Iran, “Our parents never got to see us play.”

Another member observed of Tehran, “There aren’t any hipster chicks there.”

He added, “If I go back there, they’re probably going to kill me.”

They seemed secure enough in Brooklyn, especially since New York is now the safest big city in America.

The moment then arrived when Rafie went roof to roof with the guitar case. He is said to have remained as desperate as ever to be playing again with his onetime basement music buddies.

“He wanted very badly to come back,” a senior police official says.

But he seems to have decided that was not going to happen. He instead put himself in keeping with a darker side of American culture when he reached inside the guitar case and pulled out not his bass but a Century Arms .308-caliber assault rifle. He would have been familiar with such weapons had he undergone military training back in Iran.

Police say the bassist turned gunman stepped onto a third floor outcropping and fired through a window. He then entered and encountered 35-year-old Ali Eskandarian, a singer who is not in the band. Rafie is said to have shot him in the head, killing him.

In a third-floor bedroom, Rafie came upon 28-year-old Arash Farazmand, formerly of the Free Keys, now with the Yellow Dogs. Rafie is said to have shot him in the head as well, killing him.

In a second-floor bedroom, Rafie discovered another member of the Yellow Dogs, Soroush Farazmand, the 27-year-old brother of Arash, in bed with a laptop. Rafie allegedly shot him in the chest, making him the third to die.

Two other Iranians were huddled in the far side of a room partitioned by a sheet or a blanket. Rafie apparently fired blindly through it, hitting one in the shoulder and elbow, missing the other. He returned to the third floor and at one point also fired into room that was being rented for the night by a pair of Coast Guard reservists who had come to New York for Veterans Day. They both escaped injury.

Suddenly, a man who is said by police to be presently in the Free Keys burst from a closet where he had been hiding. He grabbed the rifle and shots were fired as he and Rafie wrestled for it.

The magazine tumbled out, but there was apparently a bullet in the chamber when Rafie retreated to the roof. The two men who had been behind the partition on the second floor had fled and called 911. A radio car pulled up to the building and the cops heard a shot up above.

Rafie was found dead on the roof, having shot himself under the chin. The rifle lay beside him. Nearby was the empty guitar case.

Two members of the Yellow Dogs had escaped the carnage, including guitarist Siavash Karampour, who was at his job as a bartender in upper Manhattan. The other surviving member is Koory Mirz, who had once remarked on how lucky they were not to have been caught playing music in Tehran.

Now, in freest Brooklyn, the guitar case was vouchered as evidence in a triple murder, followed by a suicide.

Let’s just hope the survivors can keep that music going.