Image caption A police car outside a house in Corinth, Mississippi

An arrest has been made after letters containing suspected ricin were sent to US President Barack Obama and a US senator.

The alleged sender, Paul Kevin Curtis, 45, is a resident of Mississippi.

Initial tests on the letters, identified at remote facilities, showed the presence of the lethal toxin.

The FBI has said there is "no indication of a connection" between the letters and Monday's deadly attack in Boston.

The letters addressed to the president and Republican Mississippi Senator Roger Wicker were both postmarked Memphis, Tennessee, and dated 8 April.

'Elvis impersonator'

The letters read: "To see a wrong and not expose it, is to become a silent partner to its continuance," according to US media citing intelligence sources.

Ricin Can be fatal when inhaled, ingested or - most dangerously - injected

One to three castor beans chewed by a child, or just eight seeds chewed by an adult, can be fatal

The toxin is part of the waste produced when castor oil is made Q&A: What is ricin?

They were reportedly 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 being sent to an accredited laboratory for further analysis, the FBI said, with results expected in 24 to 48 hours.

Mr Curtis' relatives 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.

Earlier, police questioned a man near the US capitol who had a backpack containing sealed envelopes, but he was not taken into custody.

Reports of suspicious packages and envelopes also led to areas within two Senate office buildings being cleared temporarily.

Media playback is unsupported on your device Media caption Jay Carney, White House spokesman: "They will be conducting further tests"

Meanwhile Democratic Senator Carl Levin said an aide had received a suspicious-looking letter and that the authorities were investigating. The staff member had no symptoms, Sen Levin said in a statement, but was staying overnight in hospital as a precaution.

All congressional mail has been sorted and tested off-site since letters laced with anthrax were posted to two senators in 2001.

A spokesman for the Secret Service, which protects the US president and his family, said it was liaising with the US Capitol Police and the FBI to trace the origins of the letters.

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.