WASHINGTON (WJZ) — Letters sent to President Barack Obama and a U.S. senator tested positive for poisonous ricin in preliminary checks. An arrest has been made. Law enforcement sources say the suspect is Paul Kevin Curtis from Tupelo, Miss.

Adam May is at the mail screening facility in Landover where the letters were intercepted.

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This is a very delicate and dangerous operation. Crews are still on the scene and have been there all day long. They’re trying to make sure that additional letters that may have been tainted with ricin don’t make their way to Capitol Hill.

Hazardous material teams in Prince George’s County comb through a federal mail processing facility in Hyattsville. They’re looking for letters and packages that may be tainted with the potentially fatal toxin ricin.

“It’ll keep you from breathing. Ultimately, you won’t be able to breathe. There is no known antidote for it,” said Marc Bashoor, Prince George’s County Fire Department.

One letter that tested positive was addressed to Senator Roger Wicker of Mississippi. Then, the Secret Service intercepted another letter. It was addressed to President Obama. It set off automatic poison detection alarms.

“Mail sent here is screened. These tests are undertaken at remote sites to mitigate the risk both to those recipients and to the general population,” said White House Press Secretary Jay Carney.

The FBI says the letters were both postmarked from Memphis and dated April 8. News reports say both letters were signed, “I am KC and I approve this message.” The Senate Sergeant at Arms has instructed lawmakers not to accept sealed envelopes unless they’re delivered by a Senate postal employee or government courier. Mail delivery to the Capitol has been suspended for the next two days as a precaution.

Postal workers at the facility are highly trained in how to deal with these situations. So far, according to Prince George’s County Police, no one has reported any illness.

Derek Valcourt spoke with Maryland congressmen about this disturbing development.

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The source of those ricin letters sent to the president and a U.S. Senator from Mississippi remain under investigation. Earlier Wednesday, two Senate office buildings had to be temporarily evacuated as a result of scares over suspicious packages and suspicious letters. Several Congressional offices across the country have also reported suspicious packages.

Tensions and security were already high in the nation’s capital because of the Boston bombings. Now, news that two letters have preliminarily tested positive for ricin has added to the alarm.

“When something like this happens, we get immediate information,” said Maryland Senator Ben Cardin.

Senator Cardin has an office just down the hall from Senator Roger Wicker, an intended recipient of one of the ricin letters. He says all Congressional staff are taking precautions and alerting authorities to suspicious mail or packages. WJZ found security even checking paperwork of people entering the Senate office buildings.

“It is somewhat unnerving when you know there are people trying to cause that type of harm,” Cardin said.

“We have to protect our staff,” said Senator Dutch Ruppersberger.

Ruppersberger praises the postal workers who helped intercept the ricin letters and the police who are now investigating.

“We’re going to make sure we find out who did this and why,” Ruppersberger said.

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Again, mail service to Capitol Hill is suspended for the next few days.