"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," Director of National Intelligence John Ratcliffe said in a surprise news conference Wednesday evening.
Ratcliffe also announced that Iran was separately behind a series of threatening emails that were found to be sent this week to Democratic voters, which he said was "designed to intimidate voters, incite social unrest and damage President Trump."
Alireza Miryousefi, spokesman for the Iranian Mission to the U.N., denied the allegations to ABC News.
"Unlike the U.S., Iran does not interfere in other country's elections. The world has been witnessing U.S.'s own desperate public attempts to question the outcome of its own elections at the highest level," he said. "These accusations are nothing more than another scenario to undermine voter confidence in the security of the U.S. election, and are absurd. Iran has no interest in interfering in the U.S. election and no preference for the outcome. The U.S. must end its malign and dangerous accusations against Iran."
Florida law enforcement and the FBI previously had said they were investigating the threatening emails allegedly sent from outside the United States to registered Democrats. The emails claimed to be from by a member of the Proud Boys, according to authorities, something the group denies.
According to sheriffs' offices in both Brevard County and Alachua County, the sender claimed to belong to the "alt-right" group and said they were in possession of a voter's personal information.
The Brevard County Sheriff's Office said the email addressed the voter by name, then stated: "We are in possession of all your information. You are currently registered as a Democrat and we know this because we have gained access into the entire voting infrastructure. You will vote for Trump on Election Day or we will come after you."
President Donald Trump in the first presidential debate stirred controversy when, in response to a question about whether he condemned white supremacists, he told the "Proud Boys" to "stand back and stand by." Many argued the moment could serve to energize fringe members of the group to carry out acts of violence against opponents of the president.
"Iran has been a savvy follower of U.S. politics and saw the 'Proud Boys moment' in the last debate as an opportunity to build a narrative that would cast Trump supporters as threatening violence against Democratic voters," a senior administration official told ABC News, reacting to Ratcliffe's statement that Iran's actions were in part to damage Trump.
Google said spam filters stopped 90% of the emails sent as part of Iran's alleged election interference.
“We and others have seen evidence that an operation linked to Iran sent inauthentic emails to people in the U.S. over the past 24 hours," Google said in a statement overnight. "For Gmail users, our automated spam filters stopped 90% of the approximately 25,000 emails sent. Additionally, this morning we removed one video file on Drive and one video on YouTube with fewer than 30 views, and terminated the associated Google accounts. We referred the matter to the FBI and will continue to work with law enforcement and others in the industry to identify and remove any related content.”
Ratcliffe additionally accused Iran of distributing content including a video implying individuals "could cast fraudulent ballots, even from overseas."
"This video and any claims about such allegedly fraudulent ballots are not true," Ratcliffe said. "These actions are desperate attempts by desperate adversaries. Even if the adversaries pursue further attempts to intimidate or attempt to undermine border confidence, know that our election systems are resilient and you can be confident your votes are secure."
FBI Director Christopher Wray separately appeared at the news conference to assure Americans that the bureau will not tolerate attempts at foreign interference in the U.S. election, and would alert the American people when appropriate when it discovers such activity.
"When we see indications of foreign interference or federal election crimes, we are going to aggressively investigate and work with our partners to take appropriate action," Wray said. "You should be confident that your vote counts. Early, unverified claims to the contrary should be viewed with a healthy dose of skepticism."
The White House reacted late Wednesday, taking a shot at former President Barack Obama and the president's election opponent in the process.
"Unlike the Obama-Biden Administration, President Trump has and will always put America First," White House press secretary Judd Deere said in a statement. "He has directed the FBI, DOJ, and defense and intel agencies to proactively monitor and thwart any attempts to interfere in US elections, and because of the great work of our law enforcement agencies we have stopped an attempt by America’s adversaries to undermine our elections."
The announcement followed a joint statement by the Senate Intelligence Committee chairs Marco Rubio and ranking member Mark Warner concerning election security.
"Our adversaries abroad seek to sow chaos and undermine voters' belief in our democratic institutions, including the election systems and infrastructure that we rely on to record and properly report expressions of the voters' will," the statement said. "They may seek to target those systems, or simply leave the impression that they have altered or manipulated those systems, in order to undermine their credibility and our confidence in them."
Former top Department of Homeland Security official and ABC News contributor John Cohen said the announcement shows "we are not doing enough to protect our election process."
"Tonight we have learned that foreign entities were able to access voter registration information and send intimidating emails to American voters that were intended to influence how they vote," Cohen said. "The president needs to take this threat seriously, stop spreading Russian disinformation and he needs to tell his allies on Capitol Hill to stop chasing conspiracy theories and instead start focusing on protecting America from foreign attacks."
ABC News' John Santucci and Kirit Radia contributed to this report