St. Paul police are investigating who may have sent threatening and racist messages to Mayor Melvin Carter in the days leading up to a contentious vote on organized trash collection.

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

The ominous call marks the third racially motivated threat this month against the city's first black mayor. It comes less than two weeks after his office reported receiving hate letters that contained newspaper clippings about the upcoming trash referendum annotated with racial epithets in black marker.

"We do take it seriously," Ernster said. "Many community members voice concerns with political officials. This language takes a different path."

Ernster declined to say whether the threats have prompted increased security for Carter but noted that he does have a police officer assigned to him, standard practice for many years.

Since taking office, Carter has received at least six threatening letters and voice mails, authorities said. But recent messages are thought to be the first to include racist rhetoric.

One letter addressed to Carter said: "This is what we get for voting a [racial epithet] boy." Another envelope sent in June read, "This is B.S. you [expletive] [racial slur]." It contained no return address.

All three letters appear to be from the same person, based on the tone, language, topic and the way they were sent, police said.

Asked whether he's experienced other instances of racism since taking office, Carter said he doesn't "keep score."

"I'm not going to dwell on nasty experiences," he said Wednesday evening. "And I'm not going to give oxygen" to those who might pursue such an approach.

"Even when we disagree, we can do it with respect and civility."

Former Mayor Chris Coleman said he doesn't recall ever having to file a police report because of a direct threat of violence.

After an indoor smoking ban passed during his first term, angry residents sometimes threatened Coleman in bars and flicked lighters in his face, he said. He and his family were provided additional protection when St. Paul hosted the 2008 Republican National Convention.

"It isn't unusual to have heightened security concerns as a mayor, but I put this into a whole different category," said Coleman, who is white. "What Melvin is dealing with is far beyond anything I dealt with. … I'm deeply concerned by the increasingly out-of-control rhetoric that leads to out-of-control actions."

Earlier threats against Carter appear to be tied to politically charged issues. Several came after his decision to cancel St. Paul's annual July 4th fireworks display.

Another case involved threatening phone calls by a St. Paul man enraged about the city's failure to remove a homeless encampment near his house. Jeffrey K. Weissbach, 62, was charged with making terroristic threats after allegedly leaving a voice mail at City Hall in July saying, "It's all-out war and I will hunt you down and kill you like a dog."

"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," said Peter Leggett, Carter's communications director. "While we don't publicize our office's security measures, we … are diligent in our steps to ensure the safety of the mayor and our staff."

The hot-button trash collection issue has angered residents on both sides, drawing ire often directed at Carter.

Until a year ago, residents could choose their own haulers — or none at all — but under the current system, property owners are required to pay for trash collection and are assigned a hauler based on location.

Opponents have argued that organized collection is more expensive and fails to reward those who produce little or no waste. Supporters, including a majority of the City Council and the St. Paul DFL, have countered that the system reduces illegal dumping and truck traffic on city streets.

Last week's Minnesota Supreme Court ruling sets the stage for a stark choice Nov. 5: Keep the current system of paying quarterly bills for garbage collection, or pay for it through a hefty increase in property taxes.

If voted down, it could leave the city holding a $27 million bill for garbage pickup.

Staff writer Emma Nelson contributed to this report. liz.sawyer@startribune.com 612-673-4648 randy.furst@startribune.com 612-673-4224