A suspicious letter potentially laced with a poison, ricin, and postmarked from Memphis, was sent to the office of Sen. Roger Wicker of Mississippi, ABC News has learned.
Senate Majority Leader Harry Reid told ABC News that Capitol offices were on high alert.
The FBI was investigating the incident, officials said.
Sources confirmed the letter was sent to Wicker, R-Miss., but did not arrive at his office on Capitol Hill. It was stopped at a mail processing facility, where, officials said, a preliminary test for ricin came back positive.
Wicker's Dirksen Capitol Hill office is closed for the evening, as it is after office hours.
Wicker released a paper statement on the investigation into the letter late this evening.
"This matter is part of an ongoing investigation by the United States Capitol Police and FBI," Wicker said. "I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe. [My wife] Gayle and I appreciate everyone's thoughts and prayers."
Aides in Wicker's office emphasized that at no point did the senator's office evacuate or close because of the threat.
"Once we get information from Capitol Hill police, we will send out more," an aide to Wicker said Tuesday evening.
Sen. Jeff Flake, R-Ariz., said all mail delivery has been stopped as a precaution.
"We are very concerned," he said.
A false positive for ricin occurs at least once each year, a homeland security official told ABC News. The letter was being tested out of an abundance of caution.
Homeland Security Secretary Janet Napolitano, on Capitol Hill, declined to comment tonight on the suspicious letter.
Wicker came to the Senate in 2007 after more than a decade in the House. He was appointed by then-Gov. Haley Barbour of Mississippi.
The reports of a poisonous letter may rekindle for some people memories from 2004, when ricin was found in the office mailroom of Senate Majority Leader Bill Frist, Republican of Tennessee.
In the weeks after the terrorist attacks of Sept. 11, 2001, letters containing anthrax were sent to Capitol Hill, which prompted major changes in security and screening protocol of all mail.
Senators were made aware of the letter sent to Wicker on Tuesday night during a closed-door briefing about the Boston attacks.
The Senate sergeant at arms, Terrance Gainer, warned senators about the letter and outlined a series of precautionary steps to be taken, including the suspension of mail to the Senate.
"The exterior markings on the envelope in this case were not outwardly suspicious," Gainer said. "But it was postmarked from Memphis, Tennessee and had no return address."
A senior Senate official told ABC News that authorities had identified and were interviewing a person of interest -- someone who frequently writes letters to members of Congress. There were no injuries, but the senior official said the event was being treated as "totally real."
Members of Congress were also taking extra security steps at their district offices in their home states.
"It rarely gets to the member before it goes through a lot of staff," said Flake, the Arizona senator. "That's a big concern, obviously, for all of us. So we are very anxious to get more details on this."
The Centers for Disease Control defines ricin as a poison that comes from castor beans and can be found in a powder, a mist, a pellet or dissolved in water.
"In the 1940s the U.S. military experimented with using ricin as a possible warfare agent," the CDC writes. "In some reports ricin has possibly been used as a warfare agent in the 1980s in Iraq and more recently by terrorist organizations."
ABC News' Pierre Thomas, Jack Date, Sarah Parnass and Michael S. James contributed to this report.