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.