One day after a letter sent to a U.S. senator tested positive for the poison ricin, federal officials on Wednesday said a second letter addressed to President Obama also contained the toxic substance. 

The discovery came as law officers briefly locked down parts of the Capitol amid reports of suspicious packages being delivered to some Senate offices, and at a time of heightened tension in Washington following Monday's attack on the Boston Marathon. 

According to the FBI, the second letter was intercepted at an off-site sorting facility, and did not come close to reaching the president. The first letter, addressed to Sen. Roger Wicker (R-Miss.), was discovered at a separate off-site facility before it reached the Capitol.

The FBI said in a statement Wednesday that the letter to Obama contained a "granular substance that preliminarily tested positive for ricin." Initial ricin tests are often unreliable, so the mail has been sent to Maryland for more thorough testing that will take one or two days.

The FBI was quick to note that it has no reason to believe that the ricin letters are in any way related to the Boston bombing.

"There is no indication of a connection to the attack in Boston," the statement says.

According to the Associated Press, who obtained an FBI bulletin detailing the letters, they were both postmarked Memphis, Tenn., and contained the same cryptic message.

"To see a wrong and not expose it, is to become a silent partner to its continuance," the letters reportedly state. "I am KC and I approve this message." 

On Tuesday, Sen. Claire McCaskill (D-Mo.) told reporters that investigators already had a suspect. She added that the letters had been sent by someone who frequently writes to Congress, though she did not divulge a name. 

Ricin is a potentially deadly poison that prevents cells from producing proteins, thus causing them to die. In 2003, multiple letters sent to lawmakers tested positive for ricin, though no one was harmed. 

As for the suspicious packages, authorities issued an all-clear after briefly evacuating parts of the Capitol. Politico, citing a Capitol police officer, said authorities had spoken with a person spotted delivering mail to several offices. Tests later confirmed that the packages were harmless, though it's as yet unclear exactly what transpired. 

From the Washington Post's Aaron C. Davis and Sari Horwitz:

Authorities later reopened the Hart atrium, without immediately disclosing what — if anything — had been found.

U.S. Capitol Police officials are "talking with someone," and reports that the man has been arrested are incorrect, aides at the agency said. They would not immediately confirm whether the man had letters in his backpack or say precisely where he was stopped. [Washington Post]

Also on Wednesday, Sen. Carl Levin. (D-Mich.) issued a statement saying a staffer at his Michigan office had received a suspicious letter there. 

"The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating," the statement reads. "We do not know yet if the mail presented a threat. I'm grateful for my staff's quick response and for government personnel at all levels who are responding."

And it was reported that Sens. John McCain and Jeff Flake, both Republicans from Arizona, had received suspicious letters at the offices in their state.