Barack Obama has called the husband of Jo Cox to offer condolences on behalf of the American people, the White House has said.
The President, who is on a tour of several US national parks with his family, spoke on the phone with Brendan Cox while he was travelling on Air Force One.
Mrs Cox had worked on Mr Obama's 2008 presidential election campaign before she became MP for Batley and Spen.
The White House said in a statement: "President Obama offered his sincere condolences on behalf of the American people to Mr Cox and his two young children, as well as to her friends, colleagues and constituents.
"The President noted that the world is a better place because of her selfless service to others, and that there can be no justification for this heinous crime, which robbed a family, a community, and a nation of a dedicated wife, mother, and public servant."
Mrs Cox was shot and stabbed in the street outside her constituency surgery in Birstall, near Leeds in West Yorkshire, on Thursday.
On Friday, Hillary Clinton, the presumptive Democratic presidential nominee, said she was "horrified by the assassination" of the Labour MP and called her a "rising star".
"It is cruel and terrible that her life was cut short by a violent act of political intolerance," she said.