1:10 p.m. (EST): A third letter, this one sent to Senator Joe Manchin (D.-W.V.), has been found to contain ricin.

12:45 p.m. (EST): FBI spokesman Paul Bresson has confirmed that the substance found in the letter to the president was ricin.

The Secret Service says that a letter containing a suspicious substance and addressed to President Obama was received on Tuesday at a White House mail facility. This comes just after a letter sent to Senator Roger Wicker (R.-Miss.) was found to contain the poison ricin.

NBC is reporting that the letter to the president also contained ricin and was sent by the same person. The substance in the letter to the president has thus far only been described as "suspicious" by Secret Service spokesman Brian Leary

Some are wondering if the bombings Monday at the Boston Marathon are connected as well, in some sort of organized series of terrorist actions, but there is no evidence of that at this time.

Both letters arrived on Tuesday, at different mail-receiving stations. The letter to Sen. Wicker came from Memphis, Tennessee, but had no return address. At this time, the Secret Service, the FBI and the Capitol Police are all investigating.

Ricin has no known antidote and can kill, if inhaled, within 36 hours. Merely touching it, though, is not fatal, but may cause a rash.

[Reuters, CNN, NBC]