WASHINGTON—Law enforcement officials say a second suspicious letter that has been received in Washington was actually directed to the White House, not the Senate.

Two officials told The Associated Press on Wednesday that the letter was being treated in the same manner as a separate one sent to Mississippi Sen. Roger Wicker and was undergoing field tests.

The officials spoke on the condition of anonymity because the investigation is ongoing.

The U.S. Secret Service said it intercepted a letter addressed to President Barack Obama that contained a "suspicious substance."

The letter to Wicker, a Republican, was intercepted at a Senate mail facility just outside Washington and has tested positive for ricin. Sen. Claire McCaskill has said authorities have a suspect in mind in that case, though no one has been charged.