Iran is behind threatening, spoofed emails sent to voters, the officials said, but there was no indication that any votes themselves had been altered.

Iran and Russia have both obtained American voter registration data, top national security officials announced late on Wednesday, providing the first concrete evidence that the two countries are stepping in to try to influence the presidential election as it enters its final two weeks.

Iran used the information to send threatening, faked emails to voters, said John Ratcliffe, the director of national intelligence, and Christopher A. Wray, the F.B.I. director, in an evening announcement from the bureau’s headquarters. Intelligence agencies had collected information that Iran planned to take more steps to influence the vote in coming days, prompting the unusual timing of the briefing as an effort to deter further action by Tehran.

There was no indication that any election result tallies were changed or that information about who is registered to vote was altered, either of which could affect the outcome of voting that has already begun across the United States. The officials also did not claim that either nation hacked into voter registration systems — leaving open the possibility that the data was available to anyone who knew where to look.

The voter data obtained by Iran and Russia was mostly public, according to one intelligence official, and Iran was exploiting it as a political campaign might. Voters’ names, party registrations and some contact information are publicly available. That information may have been merged with other identifying material, like email addresses, obtained from other databases, according to intelligence officials, including some sold by criminal hacking networks on the “dark web.”

“This data can be used by foreign actors to attempt to communicate false information to registered voters that they hope will cause confusion, sow chaos and undermine your confidence in American democracy,” Mr. Ratcliffe said.