More than one hundred security researchers and experts signed on to a letter sent to member of the United States Congress to warn of their belief that not enough has been done to protect against potential threats to state and federal elections.

The letter, published Wednesday as a Senate Intelligence Committee hearing on Russian interference during the 2016 U.S. presidential election, argues many states are unprepared to respond to cybersecurity risks that may arise during upcoming election.

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The signatories laid out three primary suggestions for securing the electoral process and prevent against any potential tampering that may occur.

First, the experts called on election officials to establish voter-verified paper ballots as the “official record of voter intent.” Doing so would require phasing out paperless voting machines that offer no way to verify if a vote tallied by the system corresponds to the vote intended to be cast by the voter.

Additionally, the researchers who signed on to the letter hope to see the implementation of cyber safeguards to protect against internet-related security vulnerabilities. Included in the recommendations were the creation of firewalls between voter registration, ballot delivery and election management systems.

The group also called for the creation of layered backup systems to prevent against corruption of vital data and information so even if a system were to be breached, a previous version of the information would be secure and accessible.

Read: Did Russia Hack The US Election? Senator Warns Russian Cyberattack Worse Than Reported

Reviews to ensure compliance with the Department of Homeland Security’s recommendations for security—including penetration testing, network scanning and detection methods—were also advised. Interestingly, the group also suggested discouraging any sort of online voting even in states where it is legal, as those ballots are more likely to be subject to attack.

Finally, the security experts asked for federal elections to require a robust audit upon completion before the results are certified. The proposed review would require a comparison of random voting system samples compared to hand counts of the votes and public oversight for all audits—including publishing the results of the audits in detail.

The letter, which included signatories like PGP creator Phil Zimmermann and computer science professors at a number of institutions across the U.S., was delivered as officials from the Department of Homeland Security spoke to Congress about Russian interference during the 2016 presidential election.

Samuel Liles, the DHS’ acting Director of Cyber Division of the department's Office of Intelligence and Analysis told the committee the intelligence community concluded 21 states "were potentially targeted by Russian government-linked cyber actors."

Liles did not disclose which states were included in the 21 targeted. International Business Times reached out to the DHS for more details but did not receive a response.

Liles said only a small number of the attempted intrusions into the election systems were successful, and none are believed to have led to the manipulation of any results. According to Liles, the U.S. election system is likely to detect any attempt to change votes and said the intelligence community has a "very high level of confidence" in that conclusion.