American intelligence agencies have told the White House they now have “high confidence” that the Russian government was behind the theft of emails and documents from the Democratic National Committee, according to federal officials who have been briefed on the evidence.

But intelligence officials have cautioned that they are uncertain whether the electronic break-in at the committee’s computer systems was intended as fairly routine cyberespionage — of the kind the United States also conducts around the world — or as part of an effort to manipulate the 2016 presidential election.

The emails were released by WikiLeaks, whose founder, Julian Assange, has made it clear that he hoped to harm Hillary Clinton’s chances of winning the presidency. It is unclear how the documents made their way to the group. But a large sampling was published before the WikiLeaks release by several news organizations and someone who called himself “Guccifer 2.0,” who investigators now believe was an agent of the G.R.U., Russia’s military intelligence service.