However, the police now appear to be investigating money-related disputes. The local news media have focused on a four-year-old clash between Mr. Shiroo and the city of Nagasaki over a car accident. In that dispute, Mr. Shiroo visited City Hall about 30 times from 2003 to 2005 to demand up to 2.7 million yen, or about $23,000, for damages to his car that he claimed were from an accident at a city construction site, city officials said.

Lawyers said such requests for reimbursement were a common form of extortion. However, the small amount of money involved, and the fact that Mr. Shiroo had not raised the matter with city officials for two years, made many wonder if there was not some larger, hidden issue. Adding to the mystery was the fact that Mr. Ito was in the final stretch of campaigning for re-election in Sunday’s mayoral election, which he was widely expected to win.

“We really have no idea what was the reason” for the slaying, said Yoshinobu Hashida, chief of the city’s personnel section, which advises city officials on how to deal with gangster threats. “There’s no clear-cut explanation.”

Another possibility is a dispute over public works projects, which lawyers and other organized crime experts call a common source of revenue for criminal groups. A letter signed by Mr. Shiroo and sent to a Tokyo-based broadcaster just before the shooting reportedly expressed anger at Nagasaki for denying a contract to a construction company with links to him. On Thursday, the broadcaster, TV Asahi, turned over the letter to the Nagasaki police.

It is also unclear why Mr. Shiroo chose the mayor as his target. Mr. Shiroo did not name the mayor during the traffic accident dispute, said city officials, and the TV Asahi letter reportedly mentioned him only briefly. Toshiaki Hayashida, the mayor’s chief secretary, said he believed that Mr. Ito had never met Mr. Shiroo.

Some experts said the killing might have been an act of desperation by gangsters, who have come under increasing pressure as Japanese companies and local governments have stepped up efforts to end dealings with organized crime. In recent years, Nagasaki has passed new regulations requiring companies to report all contact with organized criminal groups, or be barred from bidding on public works contracts.

Organized crime has also felt the pinch as Japan has reduced public works spending to rein in runaway budget deficits, say experts like Takashi Ozaki, a lawyer in Tokyo specializing in organized crime cases. Criminal groups have responded by increasingly turning to violence to intimidate local officials, Mr. Ozaki said.