Police in Minnesota are investigating racist and threatening messages sent to St. Paul Mayor Melvin Carter ahead of a contentious vote.

Carter has been receiving the messages as a result of an upcoming vote for changes to the trash collection that could increase taxes. One of the most recent messages was an anonymous voicemail left to his office Monday that involved a caller reportedly telling Carter that she should install “bulletproof windows” on his house. This is on top of letters containing threatening and racist language aimed at St. Paul’s first African-American mayor.

Police have been investigating the messages but have been unable to turn up any leads as to where they came from or who sent them.

“While the two police reports over the past week relate to the garbage lawsuit, it is not uncommon for the Mayor’s Office to receive calls or letters that are reported to law enforcement,” Carter’s communications director Peter Leggett said in a statement. “While we don’t publicize our office’s security measures, we take all threats seriously, and are diligent in our steps to ensure the safety of the mayor and our staff.”

Carter, 40, a Democrat, was elected in November 2017 by a wide margin.

Residents in St. Paul could choose whether or not they wanted a trash hauler and who it was. However, that changed a year ago after it was decided property owners had to pay for trash hauling and would be assigned a hauler based on location.

Opponents of the change argued that it makes the system expensive and doesn’t reward anyone who produces little trash. However, supporters of the change countered by saying it will reduce illegal dumping and open up traffic.

The vote is currently scheduled for Nov. 5 where voters will choose between keeping the current system of quarterly billing or pay for it through a tax hike.

garbage truck
Emergency services personnel walk past a refuse truck that crashed into pedestrians in George Square, Glasgow, in Scotland Dec. 22, 2014. Reuters/Stringer