RALEIGH, N.C. (AP) — North Carolina won't clear voting-machine makers to sell their systems to county elections boards until it learns more about who owns them, the state's elections board chairman said Friday.

The decision comes amid worries of foreign election interference that have grown since special counsel Robert Mueller's April report into Russian efforts to sway the 2016 presidential election.

Mueller's report "essentially says everybody should be concerned about this and everybody should be looking harder at a lot of these things to make sure we're protected as best we can be," said Robert Cordle, the head of the state elections board. "It's just a matter of doing our due diligence now to make sure there are no problems."

The state board is giving the three companies that have already passed several rounds of screening until June 21 to disclose anyone holding a 5 percent or greater interest in their company, their parent company or any subsidiaries.

Counties would be able to buy new voting systems — which could be in use for a decade — from approved companies.

Maryland officials learned last year that a company maintaining their election infrastructure did not disclose that it was being financed for more than two years by a venture fund whose largest investor is a Russian oligarch. A U.S. Department of Homeland Security team found no evidence of intrusion on the state's election system, which was hosted and maintained by Annapolis, Maryland-based ByteGrid.

Peering into who is behind voting-equipment makers is a long overdue step that few states have taken, said Lawrence Norden, an expert on voting machines with the Brennan Center for Justice at New York University's law school.

"We know very little about voting system vendors, and this is a crucial piece of information," he wrote in an email.

After November, North Carolina will require touchscreen-only voting systems to be replaced with equipment that produces a paper record. The change will affect machines in about a third of the state's counties.

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Omaha, Nebraska-based Election Systems & Software, currently the state's only certified voting systems vendor, is one of the three companies seeking state approval again this year. The others are Boston-based Clear Ballot and Austin, Texas-based Hart InterCivic.

The move comes a week after federal authorities said they would finally conduct a forensic analysis of voter check-in software that failed on Election Day in 2016 in Durham County to see if Russian military hackers who targeted the software provider may have tampered with registration information to disrupt voting. The Homeland Security Department's analysis of laptops used in Durham County — launched after a renewed request by state elections officials — is the first known federal probe of voting technology that malfunctioned during the 2016 election, when Russian hackers infiltrated election systems in several states.

Mueller's report described how Kremlin-backed cyber spies installed malware on the network of a company that "developed software used by numerous U.S. counties to manage voter rolls." The company's name was redacted, but executives of Tallahassee, Florida-based VR Systems said Mueller was probably referring to them. VR Systems disputes that it was hacked.

VR Systems software was used in Durham County. State and local officials said previously they found no indication that the software system was hacked and blamed the Election Day trouble on poorly trained poll workers and inadequate computer maintenance.

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