Courtney Pedroza/Getty Images

In a letter to Maricopa County officials on Thursday, Arizona Secretary of State Katie Hobbs said she has "grave concerns" over the "security and integrity" of voting machines that were turned over to private contractors hired by the Republican-led state Senate to audit the November presidential election.

Using subpoenas, the state Senate was able to get the 2.1 million ballots cast in Maricopa County during the election, as well as voting machines and personal information on voters. They hired the Florida firm Cyber Ninjas, which has no election experience, to run the audit, giving them unmonitored access to everything.

Hobbs said Cyber Ninjas has "failed to provide full transparency into what they did with the equipment," and she finds it alarming that "the chain of custody, a critical security tenet, has been compromised and election officials do not know what was done to the machines while under Cyber Ninjas' control." After speaking with cyber security experts, Hobbs has determined the safest course of action is for the county "not to re-deploy any of the subpoenaed machines that it turned over to the Senate in any future elections."

Maricopa County leases its voting machines from Dominion Voting Systems, and is now in the middle of a three-year, $6.1 million lease agreement, The Arizona Republic reports. Hobbs said considering the "potential impact of decommissioning the subpoenaed equipment, including on taxpayer dollars and county operations, my office did not reach this decision lightly," but her concerns are heavy enough that she believes "the county can agree that this is the only path forward to ensure secure and accurate elections in Maricopa County in the future."