Federal prosecutors yesterday filed criminal charges against Paul Kevin Curtis, an Elvis impersonator from Mississippi who arrested a day earlier in the FBI’s investigation of letters believed to have contained the deadly poison ricin.

A criminal complaint filed in US District Court for the Northern District of Mississippi charges Curtis with threatening to harm President Barack Obama and making other threats through the Postal Service.

Three letters believed to contain ricin were addressed to a US senator, the White House and a Mississippi official, the FBI said in a statement on Wednesday.

The poison scare put Washington on edge the same week that bombings at the Boston Marathon killed three people and injured 176 on Monday, but the FBI said there was no indication the incidents were connected.

An affidavit from the FBI and the Secret Service filed in court on Thursday said that all three letters contained the same eight-line message.

“Maybe I have your attention now / Even if that means someone must die,” the letters said in part, according to the affidavit.

The letters ended: “I am KC and I approve this message.”

The initials “KC” were, in a way, self-incriminating as they led law enforcement officers to ask the staff of the US senator, Republican Roger Wicker of Mississippi, about any constituents with those initials, and the investigation then focused on Curtis, the affidavit said.

The letters themselves were still under-going more extensive testing later yesterday.

Two of them were intercepted at mail facilities before they could reach federal officials.

However, the third letter was delivered to a Mississippi state judge, the affidavit said.