The Senate Intelligence Committee released the first installment in a series of reports stemming from its investigation into Russian meddling in the 2016 U.S. presidential election, concluding that the Russian government attacked the election systems of at least 18 states.

According to the Republican-led panel, “at least 18 states had election systems targeted by Russian-affiliated cyber actors in some fashion,” with the possibility an additional three states could have been targeted, too.

Though no voter tallies or voter registration information was changed, the senators said the Russians were “in a position to, at a minimum, alter or delete voter registration data.”

“These activities began at least as early as 2014, continued through Election Day 2016, and included traditional information-gathering efforts as well as operations likely aimed at preparing to discredit the integrity of the U.S. voting process and election results,” the senators wrote in the report .

Russian actors “successfully penetrated a voter registration database” in just a small number of cases, the committee said — but cautioned that because of limited information and examination of election infrastructure by state and local officials, other activity may not have been found.

The Senate Intelligence Committee said the initial response by the Department of Homeland Security to counter the threat in summer 2016 was “not adequate,” but said DHS has made “tremendous progress over the last six months.”

The panel did concede it is still concerned about election infrastructure vulnerabilities for upcoming elections, concerns that include a lack of paper ballots as backup for many states and outdated software.

The committee has been looking into Russian interference for more than a year, and already released a list of recommendations in March.

According to Chairman Richard Burr, R-N.C., the next report in the panel’s assessment on election integrity could be released as early as next week, and the committee’s broader probe into Russian meddling is still ongoing.

“Today’s primaries are the next step toward the 2018 midterms and another reminder of the urgency of securing our election systems,” Burr said in a statement. “Our investigation has been a bipartisan effort from day one, and I look forward to completing the Committee’s work and releasing as much of it as possible. We are working tirelessly to give Americans a complete accounting of what happened in 2016 and to prevent any future interference with our democratic process.”

Sen. Mark Warner, D-Va., the committee’s top Democrat, said he is worried the U.S. is “not fully prepared” for the 2018 midterms.

“That’s one reason why we, as a Committee, have decided that it is important to get out as much information as possible about the threat, so that governments at every level take it seriously and take the necessary steps to defend ourselves,” said Warner.

Read the full report here.