Authorities have arrested a suspect in connection with those poison-laced letters intended for President Barack Obama and Republican Sen. Roger Wicker of Mississippi, the Federal Bureau of Investigation announced late Wednesday. The FBI said its agents had detained Paul Kevin Curtis at 6:15 p.m. EDT at his home in Corinth, Miss.

According to a report from Yahoo News, in a statement, the FBI identified Curtis as "the individual believed to be responsible for the mailings of the three letters sent through the U.S. Postal 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 news capped a day in which law enforcement officials announced that they had intercepted a letter sent to Obama that preliminary tests found to contain the deadly substance ricin. Officials temporarily locked down some Senate office buildings amid rising concerns of a terrorism-by-mail campaign reminiscent of the anthrax attacks that followed the 9/11 strikes.

The letter sent to Wicker was caught Tuesday. Officials said there was no evidence of a link to the bombings near the finish line of the Boston Marathon.

Asked whether the Obama and Wicker incidents were connected, a law enforcement source told Yahoo News "the letters are very similar." Other news outlets reported that they bore the same message: "To see a wrong and not expose it, is to become a silent partner to its continuance." And both were reportedly signed "I am KC and I approve this message"—a play on the required campaign advertising endorsement candidates are required to make.

Obama was never in any real danger. Since the anthrax attacks in 2001, mail addressed to the White House goes through extensive off-site screening. The same goes for that of U.S. lawmakers.

Ricin, for which there is no known antidote, is made from ground castor beans—and ground castor beans lacking the poison's potency can still trigger a positive test.

One day after the letter to Wicker was intercepted, "a second letter containing a granular substance that preliminarily tested positive for ricin was received at an offsite mail screening facility," the FBI announced. "The envelope, addressed to the president, was immediately quarantined by U.S. Secret Service personnel, and a coordinated investigation with the FBI was initiated."

Democratic Sen. Carl Levin of Michigan also disclosed that a staffer in his Saginaw office had alerted authorities after receiving a "suspicious-looking letter." "Suspicious letters" were also intercepted at Republican Sen. Jeff Flake's Phoenix office on Wednesday morning, but "no dangerous or hazardous materials" were recovered in that incident, Flake announced later in the day.

And U.S. Capitol Police investigated a fresh potential threat to lawmakers: “suspicious packages” that had led them to clear three floors in two Senate office buildings, according to a spokeswoman. "Both incidents have been cleared. We have reopened every area—business as usual," Lt. Jessica Baboulis told Yahoo News.

One of the Hill offices investigated was that of Republican Sen. Richard Shelby of Alabama.

Asked whether there was any sign that those situations were linked to the Wicker letter, Baboulis replied: “We have nothing to indicate that."