The top Democrat on the Senate Intelligence Committee is demanding that the Department of Homeland Security give state-level officials all the information it has about foreign attempts to hack into U.S. election systems.

"I am deeply concerned about the danger of future foreign interference in our elections," Sen. Mark Warner, D-Va., wrote in a Tuesday letter to Homeland Security Secretary John Kelly.

"A January 2017 Intelligence Community Assessment concluded that Russia obtained and maintained access to elements of multiple U.S. state and local electoral boards," he wrote. "While I am not aware of evidence that the 2016 voting process itself was subjected to manipulation, and have no reason to doubt the validity of the election results, we know that the DHS and FBI have confirmed two intrusions into voter registration databases in Arizona and Illinois by foreign-based hackers, though no data was modified or deleted."

He wrote that the "breadth and scope" of the intrusion tried to undermine confidence in the U.S. election system, and that DHS needs to make clear what it knows.

"I therefore urge you to work closely with state and local election officials to disclose publicly which states were targeted, to ensure that they are fully aware of the threat, and to make certain that their cyber defenses are able to neutralize this danger," he wrote. "We are not made safer by keeping the scope and breadth of these attacks secret."

On Wednesday, the committee will hold an open hearing on attempts to hack into U.S. election systems, and will feature officials from DHS, the FBI and others.