The febrile security mood in Washington rose another notch on Tuesday as it emerged that an envelope sent to Senate offices had tested positive for a poison thought to be ricin.

The envelope was addressed to the offices of Senator Roger Wicker, a Republican from Mississippi, but is understood to have been intercepted before reaching Capitol Hill.

Since anthrax attacks in the post caused Congress to shut down in 2001, all mail has been diverted to an off-site facility for testing before delivery.

CNN reported that the envelope had tested positive in a first routine test, before being retested two more times, each time coming up positive. The package was then sent to a Maryland lab for further testing, according to the law enforcement source. CNN said Wicker was now being given extra security protection.

Michael McCaul, chairman of the House Homeland Security Committee, said: "This time the system worked … it tested positive but the letter did not reach Senator Wicker's office."

He said there was no evidence that the ricin incident was linked to the attack in Boston.

"Ricin is very targeted, almost assassin-like, it does not cause mass casualties so would be inconsistent with the terrorist philosophy," McCaul told CNN. "A lot of members of Congress have threats and this is one to be taken seriously. Roger is a very nice guy and I don't know why anyone would want to do this to him."

The website Politico reported that lawmakers had been told that the letter sent to Wicker's office came from an individual who frequently writes to politicians and have been told what to look for if there are more letters containing the toxic substance.

Neither US Capitol police nor Wicker's office could be reached for comment.

Senate speaker Harry Reid also confirmed the incident separately and Senator Mary Landrieu said lawmakers were informed of the fact at a closed-door briefing about the Boston Marathon bombings.

The incident will rekindle painful memories in Washington of when two Democrat senators were targeted with envelopes containing anthrax spores in a wave of attacks across the US shortly after the September 11 terrorist attacks. In total, five people were killed by the anthrax attacks and Congress was shut for weeks to allow offices to be decontaminated.

This time, the envelope testing positive for ricin follows another terrorist attack and comes as much of Washington is in a heightened state of alert following the Boston bombings. Tourists are still prevented from approaching the White House and there is a heavy security presence across the city.

Nevertheless, previous suspected ricin tests have turned out to be false alarms as a similar test response can be caused by paper byproducts, not the deadly poison. Non-toxic byproducts of the castor bean plant – the raw material for ricin – are sometimes used in making paper.

A report for prepared for Congress in 2010 warned of the potential use of ricin for terrorist attacks.

"Persons exposed to ricin exhibit different symptoms depending on the route of exposure," it said. "Ingestion of ricin causes nausea, vomiting, diarrhoea, gastric haemorrhaging and shock. With a sufficient dose, death occurs within three to five days. Injection of ricin produces severe internal bleeding and tissue death, which can result in the collapse of major organ systems. Death often follows such a collapse."