WASHINGTON — For much of the summer, the F.B.I. pursued a widening investigation into a Russian role in the American presidential campaign. Agents scrutinized advisers close to Donald J. Trump, looked for financial connections with Russian financial figures, searched for those involved in hacking the computers of Democrats, and even chased a lead — which they ultimately came to doubt — about a possible secret channel of email communication from the Trump Organization to a Russian bank.Law enforcement officials say that none of the investigations so far have found any link between Mr. Trump and the Russian government. And even the hacking into Democratic emails, F.B.I. and intelligence officials now believe, was aimed at disrupting the presidential election rather than electing Mr. Trump.