WASHINGTON (AP) - A senator says police have a suspect in mind as they try to determine who mailed a letter that tested positive for poisonous ricin to one of her colleagues, Sen. Roger Wicker of Mississippi.

Missouri Sen. Claire McCaskill says the suspect writes a lot of letters to lawmakers. Authorities declined to comment Tuesday on a suspect or any other aspect of the investigation being led by Capitol Police and the FBI after tests indicated that a letter mailed to Wicker's Washington office contained the potentially deadly toxin.

The letter was intercepted at a Senate mail facility. Its discovery shook the U.S. Capitol, where several events had been canceled in response to Monday's deadly bombings at the Boston Marathon. There was no evidence of a connection between the two events.