Police say they have arrested a Mississippi man in connection with the poison envelopes sent to President Obama and a US senator.
Two letters containing the potentially deadly poison ricin sent to President Barack Obama and a US senator are related, the FBI has said. The letters to Mr Obama and Roger Wicker have Memphis, Tennessee, postmarks and the same date – April 8.
The bureau said both say: “To see a wrong and not expose it, is to become a silent partner to its continuance.” They are signed: “I am KC and I approve this message.”
The FBI said it was pursuing investigative leads to determine who sent the letters.
Earlier, Senator Claire McCaskill claimed authorities have a suspect in mind for the Wicker case, but no one has been charged.
Meanwhile, Senator Carl Levin said a worker at his regional office in Saginaw, Michigan, had received a suspicious letter.
He said: “The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating.
“We do not know yet if the mail presented a threat. I’m grateful for my staff’s quick response and for government personnel at all levels who are responding.”
In Arizona, a suspicious letter containing an oily substance that was sent to the Phoenix office of Senator Jeff Flake caused a brief scare before the mail was tested and found to be harmless.
The FBI said preliminary tests on suspicious substances in the two earlier letters sent to Mr Obama and Mr Wicker indicated it was ricin.
Secret Service spokesman Ed Donovan said the letter to Mr Obama, received on Tuesday, was intercepted at a facility away from the White House.
The letter to Mississippi Republican Mr Wicker was also intercepted at an off-site mail facility.
Tensions have been high in Washington and across the US since the deadly bombings on Monday at the Boston Marathon that killed three people and injured more than 170.
The FBI said there was no indication of a connection to the marathon attacks.