Test results were expected Thursday on letters possibly contaminated with the deadly poison ricin that were sent to President Barack Obama and Sen. Roger Wicker this week.
The FBI arrested Paul Kevin Curtis in connection with the case at his home on Corinth, Mississippi, on Wednesday, the department said in a statement. NBC and the TODAY Show reported Monday morning that Curtis is an Elvis impersonator.
The letters, which were discovered on Tuesday, 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 ricin.
Because initial tests can be "inconsistent," the envelopes were sent off for additional tests, an FBI statement said.
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.
Letters put the spotlight Texas chiropractor's words
The line in the letters about exposing "a wrong" comes from John Raymond Baker, a longtime Texas chiropractor, his wife said. It's been widely quoted online, but Tammy Baker sounded surprised that it was used in the letters under scrutiny in Washington.
When contacted by CNN, 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."
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.
On heightened alert
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."
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.
Beyond Washington, suspicious letters spotted
Sadie Holland, a judge in Lee County, Mississippi, told CNN on 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.
Holland said the letter included the line "Now someone must die."
Last Wednesday, the judge received and opened the 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, with results also expected on Thursday. 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."
Suspicious letters in Michigan and Arizona too
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.
Ricin is easily made
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.