The FBI identified the man as Paul Kevin Curtis of Corinth, Miss.

He was arrested at his home at approximately 5:15 CT, the FBI said in a written statement, adding he was “believed to be responsible for the mailings of the three letters sent through the U.S. Postal Inspection Service which contained a granular substance that preliminarily tested positive for ricin. The letters were addressed to a U.S. senator, the White House and a Mississippi justice official.”

The letter addressed to President Obama that field-tested positive for the poison ricin included the message, “To see a wrong and not expose it is to become a silent partner to its continuance,” according to a source familiar with an investigation of the incident.

“I am KC and I approve this message,” the letter read.

The letter was received at the remote White House mail screening facility Tuesday, according to law enforcement officials.

The facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery.

The separate Senate mail-handling facility on Tuesday also received a suspicious letter potentially laced with ricin addressed to Sen. Wicker, postmarked from Memphis. It contained the same message included in the letter addressed to the president, according to the source.

Testing on the first letter, addressed to Wicker, was incomplete but expected to be finished this evening, at the earliest, according to sources.

Field tests are often unreliable, and a false positive for ricin occurs at least once each year, a homeland security official told ABC News.

Meanwhile, the sources said, a sensor today was activated at a third federal government mail sorting facility. Investigators were culling through the mail to see whether they could identify a suspicious letter.

The facility, they said, sorts mail for the Department of Justice. The sensor was undergoing additional testing at a lab. A source cautioned that the sensor activation might involve a false positive and not be related to the other letters.

The Secret Service’s White House mail-screening facility is a remote facility, not located near the White House complex, through which all White House mail goes.

The Secret Service was working closely with the U.S. Capitol Police and the FBI in the investigation.

“The investigation into these letters remains ongoing, and more letters may still be received,” the FBI said in a statement this morning. “There is no indication of a connection to the attack in Boston.”

The FBI added: “It is important to note that operations at the White House have not been affected as a result of the investigation.

“Additionally, filters at a second government mail screening facility preliminarily tested positive for ricin this morning. Mail from that facility is being tested.”

FBI sources said anytime suspicious powder is located in a mail facility, field tests are conducted. The field and other preliminary tests in this instance produced inconsistent results. The material has been sent to an accredited laboratory for further analysis.

Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin. Those tests were in the process of being conducted and generally take from 24 to 48 hours.

The Centers for Disease Control defines ricin as a poison that comes from castor beans and can be found in a powder, a mist, a pellet or dissolved in water.

“In the 1940s, the U.S. military experimented with using ricin as a possible warfare agent,” the CDC writes. “In some reports ricin has possibly been used as a warfare agent in the 1980s in Iraq and more recently by terrorist organizations.”

Meanwhile, Senate offices were on partial lockdown today after the discovery of suspicious packages. The police investigation centered on the offices of Sens. Richard Shelby, R-Ala., in the Russell Senate Building, and Joe Manchin, D-W.Va., in the Hart Senate Office Building.

The lockdown was unrelated to the Wicker letter.

Also, the Saginaw, Mich., office of Sen. Carl Levin, D-Mich., received a suspicious-looking letter this morning. The office did not know whether it was a threat.

“The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating. We do not know yet if the mail presented a threat,” Levin said in a paper statement.

“I’m grateful for my staff’s quick response and for government personnel at all levels who are responding.”

Several senators have reported suspicious packages delivered to their district offices, with no reports of any credible threats.

But underscoring the jitters among Senate D.C. and district offices, Sen. Deb Fischer’s, R-Neb., Lincoln, Neb., office contacted police when staff found a suspicious package outside this morning.

It turned out to be a used-car part left in a bag on top of a lawn chair.

h/t ABC News.