Authorities have arrested a suspect thought to have sent potentially 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.

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.

Story continues

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."

The White House worked to tamp down fears.

"Before we speculate, before we make connections that we don’t know exist, that the FBI has made a clear statement about, we need to get the facts," press secretary Jay Carney told reporters at his daily briefing in response to a question about whether the nation is "under attack."

Levin also appeared to play down the situation in his field office.

"Earlier today, a staffer at my Saginaw regional office received a suspicious-looking letter. 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," he said in a statement. "I’m grateful for my staff’s quick response and for government personnel at all levels who are responding."

The suspicious letters and items on the Hill follow heightened tensions in the wake of Monday's deadly Boston Marathon bombings.

The arrest followed an investigation in which the FBI Joint Terrorism Task Forces in Memphis, Tenn., and Jackson, Miss., cooperated with the U.S. Capitol Police; the U.S. Postal Inspection Service; and the U.S. Secret Service, aided by the following state and local agencies: the Lee County (Miss.) Sheriff’s Office; the Prentiss County (Miss.) Sheriff’s Office; the Corinth (Miss.) Police Department; the Booneville (Miss.) Police Department; the Tupelo (Miss.) Police Department; the Mississippi National Guard 47th Civil Support Team; and the Mississippi Office of Homeland Security.

The letter sent to the president was never near the president or the White House. The screening facility that caught the president's letter on Tuesday is "not located near the White House complex," U.S. Secret Service spokesman Brian Leary noted in a statement.

"On 4-16-13, a letter addressed to the president containing a suspicious substance was received at the remote White House mail screening facility. This facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery," Leary said.

"The Secret Service is working closely with the U.S. Capitol Police and the FBI in this investigation," he said.

Senate Sergeant at Arms Terrance Gainer said in an email to all Senate staff that mail delivery would be interrupted on Thursday and Friday, and he urged exceptional precautions when dealing with incoming mail:

Offices should NOT accept sealed envelopes. All unopened items should go through screening. You should accept mail only from a uniformed Senate Post Office employee or a government courier bearing a bona-fide government ID. Should an individual attempt to deliver a sealed item to your Washington, D.C., office, please redirect them to the Congressional Acceptance Site located at 160 D Street, NE (across the street from United States Capitol Police headquarters), or if you have any doubts, call the police. We will process these items safely and deliver them after they have cleared testing. If a sealed envelope already has been delivered to your office and it is of unknown origin, you should contact the United States Capitol Police."

News of the intercepted letter to the president was reportedly announced to senators during a briefing on Tuesday evening.

This post was last updated at 9:20 p.m. EDT with the FBI statement on the arrest of the suspect.