A man has been arrested in connection with two letters suspected of containing the poison ricin sent to Barack Obama and a senator, the FBI has said.

Tensions were high in Washington at both the White House and Congress after the letters were intercepted. Initial tests showed them containing suspected traces of ricin but sent off for further lab tests for confirmation.

The FBI special agent in charge of the investigation, Daniel McMullen, said a man had been arrested on Wednesday.

Associated Press, quoting an FBI agent, separately identified the suspect as Paul Kevin Curtis, 45, of Corinth, Mississippi.

The letter to Obama was intercepted at an office away from the White House, where all mail to the president is subjected to a rigorous screening process.

The letter arrived on Tuesday, the same day that a letter addressed to US senator Roger Wicker, also thought to contain ricin, was intercepted by the mail room at the Congress. Another senator, Richard Shelby, was also the subject of an investigation after he was reported to have received a suspicious letter.

Officials said the letter to Wicker was postmarked Memphis, Tennessee.

An FBI intelligence bulletin obtained by the Associated Press quoted from the letters sent to Obama and Wicker. "To see a wrong and not expose it is to become a silent partner to its continuance." Both were signed: "I am KC and I approve this message."

Parts of Congress were cleared Wednesday after reports of other suspicious packages in two buildings housing the offices of senators, the Hart and Russell buildings. Emails were sent out to staff warning them to avoid the area. The warnings were backed up by loudspeaker announcements, but there was no evacuation.

The letters are reminiscent of those sent out in 2001 to members of Congress that contained anthrax spores. Five people died after coming into contact with the letters, which arrived in the week after the 9/11 attacks, although they were not connected to them.

In a statement on Wednesday, the FBI said there was no indication of a connection between the suspicious letters and the Boston marathon bombings. But the letters have added to the sense of uncertainty in the US in the wake of the marathon attacks.

According to the FBI, the envelope addressed to the president was immediately quarantined by secret service staff. "A second letter containing a granular substance that preliminarily tested positive for ricin was received at an offsite mail screening facility," the statement said.

Initial tests for ricin can indicate false positives, and further tests were being carried out.

Obama's press secretary, Jay Carney, asked why the White House would not rule out a connection between the letters and Boston, said the FBI had not made any such link. "It is my understanding that they have not made that connection," he said.

"The FBI has the lead in that investigation, of course, and has said in its statement [that] they will conducting further tests to determine what the nature of the substance is. Of course, there was another [letter] that was intercepted by Capitol Police that was sent to a United States senator and that also is subject to an investigation by the FBI. The president has, of course, been briefed on these letters last night and again this morning."

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."