Washington (CNN) -- Government laboratories are testing samples of a suspicious substance found in letters at off-site White House and Senate mailrooms after preliminary test results pointing to the deadly poison ricin rattled Washington, authorities said Wednesday.
White House mail handlers identified a "suspicious substance" Tuesday in a letter addressed to President Barack Obama that preliminarily tested positive for ricin, the FBI said. The same day, a similar letter addressed to Sen. Roger Wicker, R-Mississippi, tested positive for ricin -- a toxin with no known antidote, officials said.
But the FBI said initial tests can be "inconsistent," and the envelopes have been sent off for additional tests.
Meanwhile, Sen. Carl Levin, D-Michigan, says one of his home-state offices received a "suspicious-looking" letter and alerted authorities. "We do not know yet if the mail presented a threat," said Levin, the chairman of the Senate Armed Services Committee.
Filters at a government mail-screening facility preliminarily tested positive for ricin Wednesday morning, an FBI statement said, and mail from that site is being tested.
Mail for members of Congress and the White House has been handled at off-site postal facilities since the 2001 anthrax attacks. But Capitol Police were checking out reports of suspicious packages or letters in two Senate office buildings and evacuated the first floor of one those buildings Wednesday afternoon.
Police questioned a man in the area who had a backpack containing sealed envelopes, but a federal law enforcement official told CNN that authorities do not believe the man was connected to the letters found Tuesday.
In a statement issued Wednesday, the FBI said it has no indication of a connection between the tainted letters and Monday's bombings at the finish line of the Boston Marathon. But the discoveries further heightened security concerns at a time when Congress is considering politically volatile legislation to toughen gun laws and reform the immigration system.
"Monday's attack in Boston reminded us that terrorism can still strike anywhere at any time," Senate Republican leader Mitch McConnell said Wednesday. "And as yesterday's news of an attempt to send ricin to the Capitol reminds us, it is as important as ever to take the steps necessary to protect Americans from those who would do us harm."
The letter sent to Wicker had a Memphis, Tennessee, postmark and no return address, Senate Sergeant-at-Arms Terrance Gainer wrote in an e-mail to senators and aides.
A laboratory in Maryland confirmed the presence of ricin on the letter addressed to Wicker after initial field tests also indicated the poison was present, according to Gainer. However, the FBI said additional testing was needed because field and preliminary tests produce inconsistent results.
"Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin," according to the bureau. "Those tests are in the process of being conducted and generally take from 24 to 48 hours."
In a statement late Tuesday, the U.S. Capitol Police said further tests would be conducted at the Army's biomedical research laboratory at Fort Detrick, Maryland.
Sen. Claire McCaskill, D-Missouri, told reporters after a briefing for lawmakers that a suspect has already been identified in the incident, but a knowledgeable source said no one was in custody Tuesday night.
Wicker has been assigned a protective detail, according to a law enforcement source.
Postal workers started handling mail at a site off Capitol Hill after the 2001 anthrax attacks that targeted then-Majority Leader Tom Daschle, D-South Dakota, and Sen. Patrick Leahy, D-Vermont, among others.
Senators were told Tuesday that the mail facility would be temporarily shut down "to make sure they get everything squared away," McCaskill said Tuesday afternoon.
"The bottom line is, the process we have in place worked," she said, adding that members of Congress will be warning their home-state offices to look out for similar letters.
McConnell, R-Kentucky, also praised the postal workers and law enforcement officers for "preventing this threat before it even reached the Capitol."
"They proved that the proactive measures we put in place do in fact work," he said.
Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms -- an amount the size of the head of a pin -- can kill an adult. There is no specific test for exposure and no antidote once exposed.
It can be produced easily and cheaply, and authorities in several countries have investigated links between suspect extremists and ricin. But experts say it is more effective on individuals than as a weapon of mass destruction.
Ricin was used in the 1978 assassination of Bulgarian dissident Georgi Markov. The author, who had defected nine years earlier, was jabbed by the tip of an umbrella while waiting for a bus in London and died four days later.
A previous ricin scare hit the Capitol in 2004, when tests identified a letter in a Senate mailroom that served then-Majority Leader Bill Frist's office. The discovery forced 16 employees to go through decontamination procedures, but no one reported any ill effects afterward, Frist said.
Wicker, 61, was first appointed by former Republican Gov. Haley Barbour to the U.S. Senate in December 2007 after the resignation of then-Sen. Trent Lott. He was then elected to the seat in 2008 and won re-election in 2012 to a second term.
Before joining the Senate, he was a U.S. representative in the House from 1995 to 2007. Before that, he served in the Mississippi Senate.