United 93, the first Hollywood film to deal with the terrorist attacks of 11 September, was the second most popular movie in the US on its opening weekend.
United 93 dramatises events onboard a hijacked plane
It generated ticket sales of $11.6m (£6.4m) across the country, having received its premiere last week at the Tribeca Film Festival in New York.
The film dramatises events on a flight which crashed when passengers fought back against hijackers.
It was beaten to number one by Robin Williams' new comedy, RV.
Williams plays the beleaguered head of a family making a cross-country trip in a recreational vehicle which has plumbing problems.
RV earned $16.4m (£9m) on Friday, Saturday and Sunday, while the teen gymnastics drama Stick It made number three, with takings of $11.3m (£6.2m).
United 93 is the first Hollywood film to tackle the attacks of 2001
United 93's sensitive subject matter had made it difficult to forecast how it would fare in US cinemas.
Some audiences watching trailers for the film had expressed concern that it was too soon to release a drama about the events of 2001.
However, Nikki Rocco, the president of domestic theatrical distribution at Universal, said: "I think Americans have spoken loud and clear, that they were ready for a film like this."
She added the studio had not made any predictions about its box office performance because it "wasn't the first and foremost aspect of producing the film".