An Elvis impersonator from Mississippi has been charged with sending a letter containing suspected ricin to US President Barack Obama.
Paul Kevin Curtis, 45, was detained on Wednesday after letters were found addressed to Mr Obama and a senator.
An attorney for Mr Curtis said her client was surprised by the arrest and that he maintained his innocence.
The FBI earlier said there was "no indication of a connection" between the letters and Monday's Boston attack.
Mr Curtis has also been charged with threatening to harm others, the US Department of Justice said.
He appeared in court on Thursday for a brief hearing but said little, the Associated Press news agency reports.
His attorney, Christi McCoy, told reporters Mr Curtis "maintains 100% that he did not do this".
Initial tests on the letters, identified at remote facilities, showed the presence of the lethal toxin.
The letters addressed to the president and Republican Mississippi Senator Roger Wicker were both postmarked Memphis, Tennessee, and dated 8 April.
They read: "Maybe I have your attention now even if that means someone must die. To see a wrong and not expose it, is to become a silent partner to its continuance," according to an FBI affidavit released on Thursday.
The letters were signed: "I am KC and I approve this message."
The FBI said Mr Curtis also allegedly sent a third letter positively identified for ricin to a Mississippi justice official.
The contents of the letter intended for President Obama were forwarded to an accredited laboratory for further analysis, the FBI said, with results expected in 24 to 48 hours.
Mr Curtis' relatives earlier expressed shock at the news of his arrest, describing him as a natural musical performer who would impersonate Elvis Presley and Buddy Holly.
The suspect also wrote online posts about his belief that the government was running an illegal trade in human body parts, his cousin Ricky Curtis told the Associated Press.
Ricin, extracted from castor beans, is 1,000 times more toxic than cyanide.
It can be fatal when inhaled, swallowed or injected, although it is possible to recover from exposure.