Police on Tuesday intercepted an envelope that tested positive for the deadly poison ricin before it reached its intended target: a U.S. Senator.

According to CNN, who first reported on the ricin-laced envelope, the piece of mail was addressed to the office of Sen. Roger Wicker (R-Miss.), but it was discovered at an off-site facility before it made it to the Capitol. Initially there was some confusion about who was actually the intended recipient — CNN first reported only that the poisonous envelope had been discovered. In the early evening Tuesday, Senate Majority Leader Harry Reid (D-Nev.) confirmed that investigators told him the letter was addressed to Wicker.

Details are still emerging, so it's unclear who may have mailed the letter, or why. According to Politico, the FBI would not comment on the matter, and the Secret Service said that it's not involved in any investigation.

According to The New York Times, the letter was postmarked in Memphis, but bore no return address. Sen. Claire McCaskill (D-Mo.) told reporters that investigators have a suspect, and that the letter came from someone who frequently writes to Congress, though McCaskill did not provide the person's name.

The envelope first tested positive for ricin at the off-site facility, prompting a second test that also came back positive. According to CNN, the mail has now been sent to Maryland for further testing. Ricin tests are notoriously unreliable, hence the multiple rounds of testing.

Lawmakers, who had been scheduled to receive updates on the Boston Marathon attack, were also briefed on the incident Tuesday evening.

Ricin is a potentially deadly poison made from castor beans. According to the Centers for Disease Control and Prevention, ricin prevents the body's cells from creating essential proteins, eventually causing them to die. The result can be debilitating sickness or, within as little as 36 hours, death.

In 2003, authorities intercepted a letter, addressed to the White House, containing traces of ricin. The following year, another ricin-tainted envelope turned up at the Capitol. There were no reported illnesses as a result of either of those mailings.