COPENHAGEN — In the eyes of his former associates, Nedim Yasar, once the leader of a notorious Danish gang, did something much worse than just leave the criminal life: He talked about it, becoming a nationally recognized expert on gang violence.

Mr. Yasir was fatally shot on Monday night at what should have been a high point in his reformed life — he had just left a party in Copenhagen celebrating the release of a book about him, written with his cooperation. His death was big news in Denmark, lamented on the front pages of many newspapers.

“He was killed for what he said, not just because he left a brotherhood,” said Aydin Soei, an author and sociologist who interviews and writes about gang members. “The code is to leave the gang and shut up about it, but he went the other way. He stood up with his story, burning to break the chain feeding the gangs new members and to encourage others by saying there is an alternative to the gangs.”

The police in Copenhagen said they did not have any suspects and appealed for witnesses. In a statement, investigators said they were aware of an attempt on Mr. Yasar’s life at his home last year, but they had not been aware of any recent threats.