The FBI said Thursday it confirmed the presence of the deadly poison ricin in letters sent to President Barack Obama, a U.S. senator and a judge.
Earlier, an Elvis impersonator charged in the case appeared in federal court in Oxford Mississippi.
During a four-minute hearing, Magistrate Judge S. Allan Alexander ordered Paul Kevin Curtis -- who appeared in court with attorney Christi McCoy -- to remain in custody until a grand jury issues an expected indictment and a preliminary and detention hearing on April 29.
In confirming the letters tested positive for ricin, the FBI said it was "not aware of any illness as a result of exposure to these letters."
Further tests were being conducted, the FBI statement said.
Curtis, 45, a resident of Corinth, Mississippi, was charged with sending a threat to the president.
Curtis' attorney, Christi McCoy, told CNN in an e-mail that "Mr. Curtis vehemently denies the allegations against him."
A criminal complaint charged Curtis with "knowingly depositing for conveyance in the mail and for delivery from any post office any letter, paper, writing or document containing threats to take the life of or to inflict bodily harm upon the President of the United States."
The federal complaint further charges him with sending "communications addressed to other persons, and containing a threat to injure the person of others."
Curtis was to appear Friday in U.S. District Court in Oxford, Mississippi, for a detention hearing.
An affidavit in support of the criminal complaint cites the mailing of envelopes containing typewritten letters and "a suspicious granular substance" to Sen. Roger Wicker, R-Mississippi, the president and Sadie Holland, a Justice Court judge in Lee County, Mississippi.
According to the department, all three letters were typed on yellow paper and read as follows:
"No one wanted to listen to me before.
There are still 'Missing Pieces'
Maybe I have your attention now
Even if that means someone must die.
This must stop.
To see a wrong and not expose it,
is to become a silent partner to its continuance
I am KC and I approve this message"
The letter addressed to Wicker and bearing no return address was intercepted by the U.S. Senate Mail Facility in Landover, Maryland, and the FBI was alerted of it Tuesday, the affidavit says.
Three of four field tests conducted on the powder inside the envelope addressed to Wicker tested positive for a protein that later tests determined to be ricin, a lethal toxin, it says. A fourth test proved inconclusive.
Capitol Police learned from Wicker's staff that Curtis had sent similar messages to the senator and that Curtis had posted on his blog in 2010 that he was writing a novel about black-market body parts titled "Missing Pieces," the affidavit says.
Letters to Obama and Holland also cited the book, it adds.
A similar letter -- bearing no return address and postmarked April 8 -- was sent to Holland at her office in Tupelo. It too contained a "suspicious granular substance" that has yet to be tested, the affidavit says.