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

A letter addressed to US President Barack Obama apparently containing ricin has been intercepted.

Initial tests on the letter, identified at a remote facility, showed the presence of the lethal toxin.

The letter, which arrived at the facility on 16 April, is related to another laced with ricin intended for a US senator, the FBI said.

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

The Federal Bureau of Investigation (FBI) said the letters addressed to the president and Republican Senator Roger Wicker were both postmarked Memphis, Tennessee, and dated 8 April.

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.

They were reportedly signed: "I am KC and I approve this message."

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.

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?

Earlier, police questioned a man in the area 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.

Meanwhile Democratic Senator Carl Levin said an aide had received a suspicious-looking letter and that the authorities were investigating.

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.