He’s a former punk rocker who still looks the part. Bald, burly, with his rolled-up sleeves revealing elaborate tattoos, Justin Brannan hardly seems the prototype for public office.

His hardcore-punk background has been a useful origin story in his political career, as he rose in the City Council to lead its powerful finance committee, and is now running for New York City comptroller.

But it also left a public trail of interviews, offhand comments and online messages containing crass, insensitive and homophobic language that Mr. Brannan has, in recent years, apologized for using.

Now, ahead of next month’s Democratic primary for comptroller, a new trove of online messages has emerged from his past. The messages, most more than 20 years old, include a thread that cast the Columbine High School shooting in Littleton, Colo., on April 20, 1999 — which resulted in the deaths of 13 students and a teacher — as an opportunity to promote Indecision, his band at the time.

On a Dejanews Usenet online forum in 1999, an account under Mr. Brannan’s name posted that one of the high school students tied to the Columbine shooting had worn an Indecision T-shirt. The post cited a call with an unnamed news reporter.

“Yes!!! We’re famous!” the account wrote, declaring that the shooting could help the band sell records, according to a publicly accessible version of the message board, now archived on Google Groups. Mr. Brannan, who has since pushed for tougher New York State gun laws, was in his early 20s at the time of the thread.