(CNN) -- An arrest has been made in connection with possibly contaminated letters sent to President Barack Obama and Sen. Roger Wicker, the Department of Justice said Wednesday.
Paul Kevin Curtis was arrested by the FBI at his home in Corinth, Mississippi, the department said in a statement.
Discovered Tuesday, the letters were addressed to Wicker, a Mississippi Republican, and to Obama. The justice department release said a third letter was sent to a Mississippi justice official.
The letters to Wicker and Obama were stopped at a government mail-screening facility after initial tests indicated the presence of the deadly poison ricin.
Because initial tests can be "inconsistent," the envelopes have been sent off for additional tests, an FBI statement said. The FBI does not expect to receive results from the tests until Thursday, federal law enforcement sources told CNN.
The letters read: "To see a wrong and not expose it, is to become a silent partner to its continuance."
They were signed "I am KC and I approve this message," a source 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."
Beyond Washington, suspicious letters spotted
Sadie Holland, a judge in Lee County, Mississippi, told CNN Wednesday night that she received an envelope with a suspicious substance and a letter similar to the ones sent to the offices of Obama and Wicker.
Last Wednesday, the judge received and opened a typewritten letter -- postmarked from Memphis, without a return address -- that included "suspicious content," Lee County Sheriff Jim Johnson told reporters. The letter had "great consistencies and similarities" to the letters sent to Obama and Wicker, he said. Investigators were testing the contents of the letter to determine whether ricin was inside, he said.
Holland told CNN the letter originally tested negative for ricin but was being retested Wednesday. Local authorities were awaiting the test results to determine whether to file state charges, Johnson said.
"The letter was handled, the chemical was handled by several different individuals in our justice court system," Johnson said, but added that "we do not have any reason to believe that anyone's life is in danger."
Investigators are trying to determine whether suspicious letters found at Senate offices elsewhere in the country came from the same source, federal law enforcement sources said.
Sen. Carl Levin, D-Michigan, said 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 "suspicious letters" at the freshman Republican's Phoenix office, Flake spokeswoman Genevieve Rozansky said in a statement, but "no dangerous material was detected in the letters."
Phoenix Fire Department spokesman Jonathan Jacobs said the envelope contained some type of powder. The person who initially found the envelope is being treated at a Phoenix-area hospital for a pre-existing condition and stress from the event, and others in the immediate vicinity were examined as well.
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.
White House spokesman Jay Carney said the president had been briefed on the letters.
"Obviously, he understands and we all understand that there are procedures in place, as the FBI has said. There are -- there is is a process in place that ensures that materials that are suspicious or substances that are found to be suspicious at remote locations are then sent for secondary and more intense testing, and that process is under way now," Carney said.
A Texas chiropractor's words in the spotlight
While authorities in Washington investigated the letters, the wife of a Texas chiropractor said the wording in them caught her by surprise.
The phrase used in the letters is something chiropractor John Raymond Baker once said and has been widely quoted online, his wife, Tammy Bennett Baker, told CNN.
On Wednesday, she sounded surprised when told by CNN that the wording was included in the letters under scrutiny in Washington. She said she was not aware of the letters and that the phrase refers to her husband's general philosophy of care. She said their office phone started ringing frequently Wednesday afternoon, and it was "kind of freaking out our other employee."
A 2006 post on a blog for Baker's office says the comment originally was a criticism of insurance companies. Since then, the site says, it "has been a quote that has been picked up and quoted (sometimes without attribution) around the net" and "people are using it about all kinds of injustices."
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.
A law enforcement source said further tests would be conducted at the Army's biomedical research laboratory at Fort Detrick, Maryland.
After the arrest was announced Wednesday night, Wicker released a statement thanking "the men and women of the FBI and U.S. Capitol Police for their professionalism and decisive action in keeping our family and staff safe from harm." His offices in Mississippi and Washington "remain open for business to all Mississippians," Wicker said in the statement.
Mail for members of Congress and the White House has been handled at offsite postal facilities since the 2001 anthrax attacks, which targeted Sen. Patrick Leahy, D-Vermont, and then-Majority Leader Tom Daschle, D-South Dakota.
Senate mail service shut down
Senators were told Tuesday that the mail facility would be temporarily shut down "to make sure they get everything squared away," Sen. Claire McCaskill, D-Missouri, said Tuesday afternoon.
"The bottom line is, the process we have in place worked," she said.
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 Joe Johns, Jessica Yellin, Dana Bash, Tom Cohen, Terry Frieden, Deanna Hackney, Elwyn Lopez, Lisa Desjardins and Rachel Streitfeld contributed to this report.