Weapons from a faraway, long-ago war are flowing into immigrant neighborhoods here, puncturing Swedes’ sense of confidence and security. The country’s murder rate remains low, by American standards, and violent crime is stable or dropping in many places. But gang-related assaults and shootings are becoming more frequent, and the number of neighborhoods categorized by the police as “marred by crime, social unrest and insecurity” is rising. Crime and immigration are certain to be key issues in September’s general election, alongside the traditional debates over education and health care.

Part of the reason is that Sweden’s gang violence, long contained within low-income suburbs, has begun to spill out. In large cities, hospitals report armed confrontations in emergency rooms, and school administrators say threats and weapons have become commonplace. Last week two men from Uppsala, both in their 20s, were arrested on charges of throwing grenades at the home of a bank employee who investigates fraud cases.

An earlier jolt came with the death of Mr. Zuniga, who on Jan. 7 picked up the grenade, which the police believe had been thrown by members of a local gang targeting a rival gang or police officers.