WASHINGTON — The suspected hacking attempt of the Democratic National Committee’s voter database this week was a false alarm, and the unusual activity that raised concern was merely a test, party officials said on Thursday.

The blunder was caused by a lack of communication between the national committee and one of its state branches, the officials said. The Michigan Democratic Party had hired hackers to simulate an attack known as phishing, but did not inform the national committee.

The Michigan Democratic Party’s test had attributes similar to an actual hacking, said Bob Lord, the national committee’s chief security officer. When the Democratic National Committee was contacted by cybersecurity experts this week about the activity, it notified the F.B.I. out of fears that it was another Russian attempt to penetrate the committee, as Moscow did during the 2016 presidential campaign.

The jolts of panic caused by what turned out to be an intraparty action underscored the fear of another major breach of the party’s systems just weeks before the midterm elections, even as computer security has been made a priority and shored up since 2016. American intelligence officials have said there continue to be real threats from Russia to interfere in American elections.