New details about the ricin-laced letters sent to New York City Mayor Michael Bloomberg and the offices of the gun-control advocacy group last week reveal that the writer threatened Bloomberg over his stance on guns. But that won't derail the billionaire mayor's gun control efforts.

According to new details regarding the ricin letters, the writer wrote, “You will have to kill me and my family before you get my guns,” MSNBC’s Chris Jansing tweeted.

NBC New York had additional details about the threatening tone of the missives. “What’s in this letter is nothing compared to what I’ve got planned for you,” they read, according to police and law enforcement sources who spoke to the television station.

Bloomberg, 71, has been mayor of New York City since 2002. In recent years, he’s been known nationally both for his pushes to make New Yorkers healthier and his views on gun control. The mayor founded the advocacy group Mayors Against Illegal Guns and set up a super PAC last year that backs candidates who agree on his positions on guns.

The mayor said he didn’t feel threatened by the message in the letters.

"In terms of why they've done it, I don't know," he said Wednesday night, according to the Associated Press. The letters "obviously referred to our anti-gun efforts, but there's 12,000 people that are going to get killed this year with guns and 19,000 that are going to commit suicide with guns, and we're not going to walk away from those efforts.”

The letter addressed to Bloomberg, which did not include a name, were opened at a mail facility in Manhattan on Friday. A second ricin-laced letter, addressed to the Washington, D.C., offices of Mayors Against Illegal Guns, was opened on Sunday.

The postal service union said in a memo to members Tuesday that the U.S. Postal Service was “made aware” of the ricin letters on May 20. The New York Times was the first media outlet to report on the letters on Wednesday.

The memo indicated that both letters were postmarked in Shreveport, La., and that authorities are determining whether more ricin-laced letters are in the mail system. At the time of the memo, the letter sent to Mayors Against Illegal Guns was not believed to contain ricin, although it later tested positive for the toxic substance.

“We have no reason to believe that any employees are at risk from handling the [Bloomberg] letter as it passed through the mail stream,” the memo said. “The substance involved was not in a form that could be inhaled or otherwise readily ingested. Public health experts do not believe that the material in these letters presented a health risk to anyone who came in contact with them.”

The FBI is investigating who sent the letters. It’s unclear whether the missives were handwritten or typed, or whether investigators believe the same person sent both letters.

The ricin-laced letters were sent to Bloomberg and his advocacy group roughly a month after the same substance was found on letters addressed to President Barack Obama and Mississippi Sen. Roger Wicker, a Republican.

Everett Dutschke, of Tupelo, Miss., was arrested by federal agents in connection with the Obama and Wicker letters and charged with attempted use of a biological weapon.