An envelope sent to Republican Senator Roger Wicker of Mississippi tested positive for ricin, Senator Dick Durbin told reporters after a group of lawmakers were briefed by the FBI.
Ricin is a lethal poison found naturally in castor beans.
A spokeswoman for Mr Wicker referred inquiries to US Capitol Police, according to Reuters.
A spokesman for Capitol Police could not immediately be reached for comment.
More top news
A manhunt is underway after gunmen dressed in women's clothes and SWAT team uniforms attacked a boxing event in Dublin, leaving a man dead.
Thousands of junior doctors and their supporters, wearing surgical masks, will hold a demonstration over pay and conditions.
The eight-month-old female pup, which was severely underweight and dehydrated, was asleep in a dining booth.