US Secret Service agents keep watch from the roof of the White House. The Secret Service is responsible for the safety and security of president Barack Obama and his immediate family. Photograph: Jonathan Ernst/Reuters

An envelope addressed to the White House has tentatively tested positive for cyanide after two rounds of analysis.Additional testing will be necessary(...)

Police close in on the US Capitol as people run for cover after reports of a shooting. The US Capitol and the White House were placed on lockdown. Photograph: Alex Wong/Getty Images

The US Capitol was briefly locked down today after gunshots were fired outside the building following a car chase across central Washington which ende(...)

US president Barack Obama returns to the White House following a trip to Chicago yesterday. The Secret Service said that a letter addressed to Obama, that was similar to the ricin laced letter sent to New York City Mayor Michael Bloomberg, was intercepted by a White House mail screening facility. Photograph:  Win McNamee/Getty Images.

Authorities have intercepted a threatening letter addressed to US president Barack Obama that was similar to ones sent to New York mayor Michael Bloom(...)

Castor beans (shown on top) are used to produce ricin, a deadly poison, as pictured in this handout photo from the National Counter Terrorism Centre. They are similar in color and size to pinto beans (shown on bottom), but have a small pointed protrusion on the end. Photograph: National Counter Terrorism Centre/Handout/Reuters

Federal prosecutors have filed criminal charges against Paul Kevin Curtis, the Mississippi man who was arrested a day earlier in the FBI's inve(...)

 An official walks past a hazardous materials response team truck outside a mail sorting facility in Hyattsville, Maryland. An envelope addressed to US Senator Roger Wicker  tested positive for ricin at the facility where mail bound for the US Capitol is sorted. Photograph: Drew Angerer/Getty Images

The FBI arrested a Mississippi man yesterday in connection with letters sent to President Barack Obama and two other officials that are believe(...)

Investigators comb the site of the second of two bomb explosions on Boylston Street in Boston. Photograph: Eric Thayer/The New York Times

A letter addressed to US president Barack Obama containing a substance that initially tested positive for the poison ricin was(...)