A letter address to President Barack Obama was being tested for the poison ricin on Wednesday.

A law enforcement source told CNN that the letter was "very similar" to a letter sent to Sen. Roger Wicker (R-MS) that tested positive for ricin on Tuesday. The Associated Press was also told that both letters were being treated in the same manner.

A Secret Service spokesperson confirmed to CNN that the letter addressed to Obama contained a "suspicious substance." The letter was reportedly intercepted at an off-site mail facility and did not make it to the White House.

There were no indications that either letters were connected to the Monday bombings in Boston.

Update (11:50 p.m.): CNN is reporting that a third "suspicious envelope" was sent to Sen. Richard Shelby's (R-AL) office. Parts of two Senate office buildings have been cleared.

Update (11:50 p.m.): The FBI confirmed in a statement that the letter sent to President Obama initially tested positive for the poison ricin.

"A second letter containing a granular substance that preliminarily tested positive for ricin was received at an offsite mail screening facility. The envelope, addressed to the President, was immediately quarantined by U.S. Secret Service personnel, and a coordinated investigation with the FBI was initiated. It is important to note that operations at the White House have not been affected as a result of the investigation," the statement said.

The FBI noted that preliminary tests are often incorrect and "[o]nly a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin."