Washington (CNN) -- [Breaking news update at 12:25 p.m. Wednesday: 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," Levin said.]
White House mail handlers identified a "suspicious substance" -- possibly the poison ricin -- in a letter sent to President Barack Obama the same day a similar letter was found in a Senate mailroom, the Secret Service said Wednesday.
Both letters arrived Tuesday at off-site postal facilities set up after the 2001 anthrax attacks and have been sent to laboratories for additional tests, authorities said.
"A letter addressed to the president containing a suspicious substance was received at the remote White House mail screening facility," Secret Service spokesman Brian Leary said. The Secret Service, FBI and Capitol Police are investigating, he said.
The FBI said later that the envelope "preliminarily tested positive for ricin," a deadly toxin with no known antidote. But initial tests can be "inconsistent," and the envelope was sent to a laboratory for additional tests.
"Additionally, filters at a second government mail screening facility preliminarily tested positive for ricin this morning," it said. "Mail from that facility is being tested."
Capitol Police were checking out reports of suspicious packages or letters in two Senate office buildings around noon Wednesday. The first floor of the Hart Senate Office Building was evacuated shortly before noon, and police were questioning a man with a backpack in the area.
The man raised suspicion with the content of his backpack and how he responded to police questions, two Capitol Hill police officers said. The backpack contained sealed envelopes and was being x-rayed, one of the officers said.
A federal law enforcement official told CNN that authorities do not believe the man was connected to the letters found Tuesday.
The developments came a day after preliminary tests on a letter sent to the Senate indicated the presence of Further tests on that letter took place Wednesday, the FBI said.
Senate Majority Leader Harry Reid said he was told the envelope was addressed to the office of Sen. Roger Wicker, a conservative Republican from Mississippi. It had a Memphis, Tennessee, postmark and no return address, Senate Sergeant-at-Arms Terrance Gainer wrote in an e-mail to senators and aides.
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."
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.
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.
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.
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.