A letter addressed to President Barack Obama tested positive for the poison ricin and was from the same sender who mailed a letter to a senator that also tested positive, officials told NBC News on Wednesday.
The letter to Obama was intercepted at an off-site White House mail facility and was being tested further, the FBI said. A federal law enforcement official said that the letter was "very similar" to one addressed to Sen. Roger Wicker, R-Miss.
Federal officials told NBC News that they believe they know who sent the letters, but no arrest was made because authorities were waiting for further test results.
Ricin is made from castor beans and can kill within 36 hours. There is no antidote. Some threatening letters simply contain ground castor beans, resulting in a positive field test for ricin without the concentrated poison. Results from full laboratory tests are expected in the next 24 to 48 hours.
Filters at a second government mail screening facility also tested positive for ricin in preliminary screening Wednesday.
An FBI official told NBC News that the agency did not initially believe the letters were related to the attack on the Boston Marathon on Monday.
Authorities cleared the atrium of a Senate office building Wednesday and were investigating a suspicious package there. Capitol police were also investigating a suspicious package at the office of Sen. Richard Shelby, R-Ala. Shelby's staff had not been evacuated.
The FBI confirmed the preliminary positive test on the Wicker letter Tuesday. That letter was intercepted at a postal facility in Maryland that screens mail sent to Congress, and never reached Wicker's office. The senator thanked law enforcement and said an investigation was underway but did not elaborate.
Other senators were made aware of the Wicker letter during a briefing Tuesday evening on the bombing in Boston. Sen. Claire McCaskill, D-Mo., said that the person who sent Wicker the letter writes often to elected officials.
The Wicker letter was postmarked Memphis, Tenn., and had no return address.
People can be exposed to ricin by touching a ricin-laced letter or by inhaling particles that enter the air when the envelope is opened. Touching ricin can cause a rash but is not usually fatal. Inhaling it can cause trouble breathing, fever and other symptoms, and can be fatal.
Field tests are conducted anytime suspicious powder is found in a mail facility. That and other preliminary tests can produce inconsistent results. When tests show the possibility of a biological agent, the material is sent to a laboratory for more analysis.
Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin. Those tests are currently being conducted and generally take a day or two.
Kasie Hunt, Kelly O'Donnell, Richard Esposito, Mike Viqueira and Dr. Kristina Krohn of NBC News contributed to this report.