President Obama walks across the South Lawn of the White House in Washington, May 29, 2013, to the Marine One helicopter for a short trip to Andrews Air Force Base, Md. then onto Chicago for fundraising events. (Jacquelyn Martin/AP)

The FBI has intercepted a letter addressed to President Obama that may contain the poisonous substance ricin.

A spokesman in the FBI’s Washington Field Office said that a “suspicious letter” sent to Obama was intercepted at a White House mail facility on Thursday morning. The FBI is investigating the letter and does not yet know whether the letter tested positive or negative for ricin.

The Secret Service confirms that the letter appears similar to a ricin-laced letter addressed to New York Mayor Michael R. Bloomberg (I). It’s not yet clear whether the incidents are related.

“U.S. Secret Service can confirm that the White House mail screening facility intercepted a letter addressed to the White House that [was] similar to letters previously addressed to Mayor Bloomberg in New York,” the Secret Service said in a statement. “This letter has been turned over to the FBI Joint Terrorism Task Force for testing and investigation.”

On Wednesday, authorities intercepted ricin-laced letters sent to Bloomberg and a gun-control group he founded called Mayors Against Illegal Guns, which has been prominent in the recent push for stricter gun laws.

How the U.S. Postal Service checks for pathogens in mail, like ricin, sent to Congress and federal agencies.

The news comes six weeks after ricin-laced letters were sent to Obama, Sen. Roger Wicker (R-Miss.) and a Mississippi judge in the days after the Boston Marathon bombings. Police arrested a man who was later released, then arrested James Everett Dutschke of Mississippi.

Police in Washington state last week also arrested a man in connection with a death threat to a judge and two ricin-laced letters sent this month.