Updated at 5 p.m. ET
A third suspicious letter discovered this week -- this one addressed to President Obama -- has been intercepted and is being tested for the poisonous substance ricin, CBS News has learned.
Threatening letters to the president and congressional leaders are not unusual -- their mail is processed at a facility about 10 miles outside of Washington in Landover, Md. The current security protocols -- which seem to be working exactly as they were set to do -- were put in place after the 2001 anthrax scare that targeted congressional leaders.
In addition to the letter sent to Mr. Obama this week, two letters being tested for ricin were. At least one of the letters addressed to Wicker tested positive in a preliminary field test for ricin.
The envelope addressed to the White House, the FBI said in a statement Wednesday, also initially tested positive for ricin and was immediately quarantined by U.S. Secret Service personnel. A coordinated investigation with the FBI was initiated. Field tests are regularly inaccurate; therefore, the FBI is conducting more thorough tests on the letters.
At a press briefing Wednesday, White House spokesman Jay Carney referred questions about the letters to the FBI, adding, "The president has of course been briefed on these letters. He was briefed last night and again this morning."
All three letters came from the Memphis area, and all three were sent the same day, April 8. The letters, CBS News has confirmed, said in part, "No one wanted to listen to me before. This must stop. To see a wrong and not expose it is to become a silent partner to its continuance. I am KC and I approve this message."
The interception of the letters follow the Boston Marathon bombing on Monday. At a senators-only briefing on Tuesday about the bombing, senators were also briefed by Sergeant at Arms Terence Gainer about the Wicker letter that tested positive in initial tests. Sen. Claire McCaskill, D-Mo., told reporters that the senators were told the suspected letter writer writes a lot of letters to members.
"The investigation into these letters remains ongoing, and more letters may still be received," the FBI said in its statement. "There is no indication of a connection to the attack in Boston."