A letter addressed to President Obama believed to contain the deadly poison ricin closely resembles two letters sent to New York City Mayor Michael Bloomberg and his pro gun control advocacy group.

The Secret Service on Thursday intercepted the letter to the president at the White House’s mail screening facility.

The New York City Police Department said Wednesday that the letters to Bloomberg and Mark Glaze, director of the gun control group Mayors Against Illegal Guns, referenced the recent debate over the country’s gun laws.

The FBI has taken the lead on investigating the letter to Obama and is testing the letter for ricin. If it tests positive, it would be the second parcel containing ricin that has been addressed to the president since last month.