WASHINGTON, April 23 (Xinhua) -- U.S. attorneys on Tuesday dismissed charges against a suspect who had been arrested for allegedly sending ricin-laced letters addressed to U.S. President Barack Obama and two others.

U.S. attorneys moved to dismiss charges against Paul Kevin Curtis, a suspect charged for using mail to threaten the president and two others, after he was released from custody. The charges could carry as many as 15 years in prison if convicted.

The decision comes a day after authorities failed to find evidence of the poison at his house. The 45-year-old suspect was arrested last Wednesday at his residence in Corinth, Mississippi, following investigation to suspicious letters addressed to Obama, Senator Roger Wicker and a local official in Mississippi.

Curtis has pleaded not guilty and his lawyers suggested he had possibly been framed for using mail to threaten the president.

The suspicious letters addressed to Obama and Senator Roger Wicker were intercepted a week ago at off-site mail facilities for the White House and the Capitol Hill respectively and both tested positive for ricin, a highly toxic substance.