New York Mayor Michael Bloomberg was the intended recipient of a threatening letter laced with the deadly poison ricin, dpa reported according to US media reports Wednesday.
The envelope was intercepted in a city building, according to police cited by The New York Times.
A second ricin-laced letter was found in a building in Washington DC that houses an organization pushing for stricter gun controls - Mayors Against Illegal Guns.
Bloomberg was a major founder of the organization that has run pro gun control advertising around the country targetting members of Congress who have opposed tightened gun controls.
Both letters were sent from the same address, the Times reported. The letters contained threats to Bloomberg that make reference to the political debate over gun control.
Ricin is a highly lethal poison. Small amounts are enough to kill a person.