A Russian cyberattack that targeted Democratic politicians was bigger than it first appeared and breached the private email accounts of more than 100 party officials and groups, officials with knowledge of the case said Wednesday.

The widening scope of the attack has prompted the F.B.I. to broaden its investigation, and agents have begun notifying a long list of Democratic officials that the Russians may have breached their personal accounts.

The main targets appear to have been the personal email accounts of Hillary Clinton’s campaign officials and party operatives, along with a number of party organizations.

Officials have acknowledged that the Russian hackers gained access to the Democratic Congressional Campaign Committee, which is the fund-raising arm for House Democrats, and to the Democratic National Committee, including a D.N.C. voter analytics program used by Mrs. Clinton’s presidential campaign.