(CNN) -- "To see a wrong and not expose it, is to become a silent partner to its continuance."
So read the letters that touched off a poison scare across Washington after the contents of the envelopes discovered at off-site White House and Senate mail rooms showed preliminary positive results for ricin, a law enforcement source said Wednesday. The letters were signed, "I am KC and I approve this message," the source said.
Discovered Tuesday, the letters were addressed to President Barack Obama and to Sen. Roger Wicker, R-Mississippi. Because initial tests can be "inconsistent," the envelopes have been sent off for additional tests, an FBI statement said. Those results were expected later Wednesday, an FBI official told CNN.
Preliminary tests on filters at a government mail-screening facility also indicated the presence of ricin Wednesday morning, and mail from that site also was being tested, the FBI said.
Reports of suspicious packages and envelopes also came into two Senate office buildings late Wednesday morning. Capitol Police evacuated the first floor of the Hart Senate Office Building for more than an hour and questioned a man in the area who had a backpack containing sealed envelopes, but the man was not taken into custody.
"It just reminds you that with public service comes the real possibility that you could be a target," said Sen. Pat Roberts, R-Kansas. "But on the other side of it, we have an excellent police force, and I think they'll get to the bottom of it."
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.
A staffer for Arizona Sen. Jeff Flake flagged another "suspicious letter" at the freshman Republican's Phoenix office, Flake spokeswoman Genevieve Rozansky said. But U.S. Postal Inspector Keith Moore told CNN affilliate KPHO that investigators found no hazardous substance.
Phoenix Fire Department spokesman Jonathan Jacobs said the envelope contained some type of powder. The person who initially found the envelope is currently being treated at a Phoenix-area hospital for a pre-existing condition and stress from the event, and others in the immediate vicinity were being examined as well.
The line about exposing "a wrong" comes from John Raymond Baker, a longtime Texas chiropractor, his wife told CNN. It's been widely quoted online, but Tammy Baker sounded surprised that it was used in the letters under scrutiny in Washington.
She said the phrase refers to her husband's general philosophy of care. She said the office phone started ringing frequently Wednesday afternoon, and it was "kind of freaking out our other employee."
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.
Mail for members of Congress and the White House has been handled at off-site postal facilities since the 2001 anthrax attacks, which targeted Sen. Patrick Leahy, D-Vermont, and then-Majority Leader Tom Daschle, D-South Dakota.
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 Tuesday. Wicker has been assigned a protective detail, according to a law enforcement source.
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 Tuesday night that a suspect has already been identified in the incident. But a knowledgeable source said no one was in custody.
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.
Senate Minority Leader Mitch 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 it in a letter in a Senate mail room 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.
CNN's Tom Cohen, Terry Frieden, Deanna Hackney, Elwyn Lopez, Lisa Desjardins and Rachel Streitfeld contributed to this report.