Correction: An earlier version of this article, because of information received from law enforcement, incorrectly identified the suspect arrested on suspicion of mailing letters containing ricin.

Fire crews arrive at the unmarked postal building by the Beltway every few days, donning protective rubber suits and carrying away suspicious letters addressed to members of Congress. At a similarly secret building in the District, agents set aside letters once or twice a week to the president containing powder or other cause for concern. For years — ever since the anthrax attacks of 2001 — nearly every one has proved harmless.

But alarms sounded in both buildings this week, and the threats appeared real: A grainy substance tested positive for the lethal toxin ricin in letters addressed to President Obama and a quiet senator from Mississippi.

By Wednesday night, authorities had arrested Paul Kevin Curtis of Corinth, Miss., as a suspect in the ricin mailings, the FBI said in a statement. Curtis also sent a third letter to a Mississippi justice official, the FBI said. He is well known to law enforcement as a frequent letter-writer to lawmakers, two officials said.

Sen. Roger Wicker (R-Miss.), in a statement late Wednesday, thanked law enforcement officials “for their professionalism and decisive action in keeping our family and staff safe from harm.”

The ricin scare had spilled into public view in dramatic fashion earlier Wednesday, less than two days after the deadly bombings at the Boston Marathon and bringing with it an eerie echo of the waves of fear that followed the terrorist attacks of Sept. 11, 2001.

Suspicious envelopes were hand-delivered to the Capitol Hill offices of senators from Alabama and West Virginia, prompting evacuations of their staffs, and lockdowns of many more. Two other senators — from Arizona and Michigan — reported that authorities were investigating suspicious letters delivered to district offices in their home states.

In all, five senators, including some in the thick of contentious negotiations over gun-control and immigration bills, were sent into emergency mode. Another wave of anxiety swept through the Capitol just before lunchtime when a bag left in the entranceway of a Senate building brought a bomb squad racing toward Capitol Hill. Police ordered thousands of staffers and aides not to leave their offices.

After two tense hours, the package was cleared, as were two letters delivered earlier to the offices of Sens. Joe Manchin III (D-W.Va.) and Richard C. Shelby (R-Ala.).

“I’m just tickled to death everybody’s safe, that’s all I was caring about,” said Manchin, who was on the floor of the Senate lobbying for his measure to expand background checks for gun buyers when he was alerted that his staff had been evacuated after a letter had been left on the front desk of his office.

Manchin dismissed the idea that his role in the gun-control legislation, which failed late Wednesday, could have made him a target. He also shook his head at the timing, coming right after the Monday attack in Boston: “Strange time in America, isn’t it?”

Phone messages left at numbers listed for Curtis and a relative in Mississippi were not immediately returned late Wednesday.

Late Wednesday, the FBI said it was awaiting final word on whether the letters to Obama and Wicker definitely contained ricin, a poison made of processed castor beans.

Law enforcement officials said the poison was detected in initial screenings of both letters Tuesday. They said the two also came up positive in a second, similar test at a military facility in Maryland, the officials said.

More testing to verify the presence of the poison — using fluorescent antibodies, chromatography and searching for DNA — can take 24 to 48 hours, one law enforcement official said.

The initial tests can be inaccurate. In 2004, a letter to then-Senate Majority Leader Bill Frist (R-Tenn.) was believed to contain ricin but later was shown to be harmless.

As a bioterrorism agent, ricin has the advantage of being easily made and highly potent. When castor beans are crushed for oil, the compound is left behind in the mashed material, of which more than a million tons is produced around the world each year.

Aside from suicides and accidental poisonings, the only known killing by ricin was Georgi Markov, a Bulgarian journalist who was stabbed by an umbrella on a London street in 1978. The umbrella’s tip injected a tiny metal capsule containing ricin into Markov’s leg. He died three days later.

In lab tests involving monkeys, powdered ricin has been found to cause bleeding in the lungs and suffocation within days.

At a White House briefing, spokesman Jay Carney said that Obama had been notified but that he did not “have a way to characterize” the president’s reaction.

“Obviously he understands and we all understand that there are procedures in place,” Carney said. “There’s 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 underway now.”

On the question of whether there might be any link to the bombings in Boston, Carney deferred to the FBI, which said Wednesday that it did not immediately see any connection between the cases.

The letter to Obama was intercepted Tuesday at a Secret Service-run mail-sorting facility in Anacostia, according to law enforcement officials.

The FBI said in a statement Wednesday that the letter contained “a granular substance that preliminarily tested positive for ricin.” Authorities did not say when or where the letter was postmarked.

The other letter, to Wicker, was uncovered Tuesday at a Prince George’s County sorting facility where congressional mail has been screened since anthrax-laced letters were sent to Capitol Hill in 2001. The letter to Wicker was postmarked from Memphis and contained no return address or “outwardly suspicious” markings, Senate Sergeant at Arms Terrance Gainer wrote to senators in an e-mail.

Both letters were signed, “I am KC and I approved this message,” according to NBC News.

The two letters were transferred to a secondary testing facility in Maryland, according to the law enforcement officials, who spoke on the condition of anonymity because the investigation was ongoing.

A glimpse of the investigation came outside a low-slung, white-and-green postal warehouse north of FedEx Field, where county fire crews helped emergency workers suit up in white-and-yellow hazmat suits before trudging inside a yellow tent. Two black SUVs and a car bearing the insignia of the FBI Joint Terrorism Task Forces sat behind the building.

In Phoenix, a suspicious letter was discovered Wednesday at the office of Sen. Jeff Flake (R-Ariz.). By afternoon, Flake said in a statement that tests on the letter were negative for ricin.

Sen. Carl Levin (D-Mich.) also said in a statement that a staffer received “a suspicious-looking letter” Wednesday morning at the senator’s office in Saginaw.

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

Juliet Eilperin, Peter Hermann, Paul Kane, Ed O’Keefe, Sari Horwitz, Eli Saslow and Matt Zapotosky contributed to this report.

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