Letters laced with the deadly poison ricin sent to New York Mayor Michael Bloomberg and the lobbying organization Mayors Against Illegal Guns illustrate how the group has emerged as a focal point of anger for opponents of gun control.
Three letters containing an "oily substance" that turned out to be ricin were intercepted on their way to Bloomberg's office and the mayors group. A similar envelope was sent to President Barack Obama, the Secret Service confirmed on Thursday.
The letters contained warnings against taking away people's guns, stating that anyone who comes "to my house will be shot in the face," ABC News reported.
Mayors Against Illegal Guns has taken on the position as a major opponent of the powerful National Rifle Association, which has effectively advocated on behalf of gun rights for years.
There was no suggestion that the NRA, which is enormously influential in the U.S. Congress, had any connection to the ricin letters.
The group of mayors was the brainchild of Bloomberg and Boston Mayor Thomas Menino, who formed it in 2006, arguing that mayors were uniquely sensitive to gun violence since it often fell to them to comfort the families of slain police officers.
Their group was a successor to the Brady Campaign to Prevent Gun Violence, which focuses its efforts on Congress.
"Mayors Against Illegal Guns has really emerged at the forefront of the gun control movement," said Adam Winkler, a law professor at the University of California, Los Angeles and the author of "Gunfight: The Battle over the Right to Bear Arms."
"We're seeing new money come into elections from the gun control side. That's a major shift. For years, political spending on this issue has been dominated by the NRA, you might even say it's been the exclusive province of the NRA."
Although it has a handful of private donors, the bulk of the mayors group's budget comes out of the billionaire New York mayor's own fortune. Over the last year, Bloomberg has bolstered its efforts by bankrolling campaigns in support of candidates who share his views on guns and opposing those who do not.
In 2007, the NRA's news magazine "America's 1st Freedom" put Bloomberg on its cover, depicting him as an octopus under the headline "Tentacles!"
The December 14 shooting at a school in Newtown, Connecticut, which killed 20 children and six school employees, sparked a fresh debate in the United States about gun rights, which are enshrined in the Second Amendment of the U.S. Constitution.
As polls suggested a new willingness to embrace gun-control measures, notably universal background checks, the group of mayors found more city leaders taking an interest in their efforts.
In 2009, there were 522 mayors listed on the membership roster at the group's website. In the wake of the Newtown massacre that number had swelled to nearly 1,000.
The NRA also reported a surge in membership since the Newtown attack. At its annual meeting this month, it said it had taken on 1 million new members since then, bringing its total membership to 5 million people.
In April the NRA won a major victory in Congress when it beat back a proposal supported by Obama to expand background checks for gun buyers.
NRA officials did not respond to a request for comment.
Bloomberg and Menino, each of whom is in his final year in office, said the threats contained in the ricin-laced letters would not deter them from advocating gun controls.
"This is not going to stop us. They can make all the threats they want," Menino, told ABC News.
"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," said Bloomberg. "This is a scourge on the country that we just have to make sure that we get under control and eliminate."
Gun control advocates lauded the group of mayors.
"They've provided a counterweight to the clout and the money of the NRA," said Leah Barrett of New Yorkers Against Gun Violence. "For that, we're very grateful, certainly in New York."