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

Suspicious_Letters_09ae0_image_1024w FILE: Federal authorities wearing hazmat suits walk to the home of Everett Dutschke for additional investigation, Tuesday evening, April 23, 2013 in Tupelo, Miss., in connection with the recent ricin attacks. (AP Photo/Rogelio V. Solis)

A spokeswoman in the FBI’s Washington Field Office said that the Secret Service intercepted a "suspicious letter" 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.

(More: How the U.S. Postal Service screens mail for biological agents)

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 both 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.

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 earlier this month.

Updated at 3:01 p.m. This post initially stated that the FBI had intercepted the letter, when in fact the Secret Service intercepted the letter. The post has been corrected.