ST. PAUL, Minn. — The black mayor of St. Paul, Minnesota, has received racist threats amid a long-simmering dispute over how the city should collect its trash.

Police in Minnesota's capital are investigating threatening and racist messages to Mayor Melvin Carter in the run-up to a citywide vote on organized trash collection.

An anonymous caller using racial slurs left a voicemail at Carter's office on Monday, police spokesman Sgt. Mike Ernster said. The caller warned that if taxes increase, Carter will have to "pay for it" and be forced to put "bulletproof windows" in his house.

Carter also received racist hate mail earlier this month. The letters contained newspaper clippings about the upcoming trash referendum with handwritten notes in black marker. At least one of the letters has been dusted for fingerprints, Ernster said.

Carter is St. Paul's first African American mayor. The mayor's office said it takes all threats seriously. Like previous St. Paul mayors, Carter has a police officer assigned to him for security.

Carter has warned that the city could face hefty property tax hikes if voters reject the trash ordinance on Nov. 5 because the city would have no authority to make residents pay their garbage bills. If voters reject the referendum, the city could be on the hook for a $27 million bill for trash pickup, the Minneapolis Star Tribune reported.

Until a year ago, St. Paul residents could choose their own haulers or none at all. The current system requires property owners in the city of about 306,000 to pay for trash collection and assigns them a hauler based on location.

Opponents of organized collection argue that the system is more expensive and does not reward people who produce little or no waste. Supporters maintain that the system reduces illegal dumping and truck traffic on city streets.

A majority of the City Council as well as the St. Paul Democratic Party have endorsed a "yes" vote on the referendum. Last week, the Minnesota Supreme Court ruled that even if St. Paul voters reject organized trash collection, the city will have to hold up its end of a five-year contract with haulers.