Russians hacking into the U.S. voting system to influence the upcoming election is a concern of top cybersecurity officials, though differences between the states’ procedures provide some defensive advantage, according to the leader of U.S. Cyber Command.

At a Senate Armed Services Committee hearing on Tuesday, Adm. Michael Rogers, commander of Cyber Command, declined to talk about recent reports that the Russians are undertaking a secret operation to hack into the U.S. political system and ensure the outcome is favorable to Russia. Rogers said he can’t talk about the U.S. assessment in an open forum while an investigation is still ongoing.