Federal prosecutors have demanded that millions of North Carolina voter records be turned over to immigration authorities by September 25.

With just two months to go before the midterms, the summons threatened to sow chaos in the state’s election machinery, while renewing the Trump administration’s repeatedly discredited claims of widespread voting by unauthorised immigrants.

The unsealed grand jury subpoenas were sent to the state elections board and to 44 county elections boards in eastern North Carolina. Their existence became widely known after Marc E Elias, a voting rights lawyer aligned with the Democratic Party, mentioned them on Twitter.

Though the nature, scope and impetus of the federal investigation that generated the subpoenas remain shrouded in mystery, the move appeared to be part of an effort to find and crack down on any unauthorised voting by non-citizens.

Representatives of ICE and the Justice Department officials involved declined to comment on the matter.

The subpoenas were issued last week, after federal officials announced that 19 non-citizens in North Carolina had been charged with casting illegal votes in the 2016 election. Those indictments arose from an investigation by a newly created federal task force focused on document and benefits fraud in the Eastern District of North Carolina, where the 44 counties are.

Critics say that those arrests hardly constitute a wave of voter fraud worthy of such a broad demand for documents, which they said could overwhelm the budgets and staffs of small counties just when they need to be preparing for the November election. They also raised concerns about privacy. Among the prosecutors’ demands are millions of secret ballots cast by absentee and early voters whose identities could be easily traced.

Some critics also speculated that the Trump administration was looking to continue the work of its much-maligned voter fraud commission, which was disbanded in January in a blizzard of complaints and litigation after finding no evidence of significant fraud or a corrupt voting system.

It was the latest in a series of controversies surrounding the question of voter fraud, and whether Republican-backed efforts to combat it mask an effort to actually suppress voter turnout.

North Carolina has had more of these controversies than most states. In 2016, a federal appeals court struck down a state voter identification law written by North Carolina Republicans, saying that it was aimed at African-American voters “with almost surgical precision.” On Election Day this year, voters will consider a Republican-backed constitutional amendment that would require voters to show a photo ID at the polls.

The state elections board audited the 2016 election and reported in April 2017 that it had found 41 suspected cases of non-citizens casting votes.

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President Trump has nonetheless called the US voting system “rigged,” and has frequently made unsupported claims that millions of people, including many unauthorised immigrants, have been voting illegally.

When the Trump administration’s voter fraud commission imploded in January, the commission’s vice chairman — Kris Kobach, the Kansas secretary of state — said its work would be transferred to the Homeland Security Department; ICE is part of that department.

Bryan D Cox, an ICE spokesman, said that the investigation in North Carolina was “locally driven” by the office of the US attorney for the Eastern District, Robert J Higdon Jr.

In previous administrations, US attorneys routinely sought approval from senior Justice Department officials before undertaking investigations that were likely to attract national attention or cause controversy, according to Vanita Gupta, who led the department’s Civil Rights Division under President Barack Obama. “Given its atypical nature, it’s hard to imagine that main Justice was unaware” of the investigations and the subpoenas, she said.

A Justice Department official in Washington referred questions about the matter to Mr Higdon’s office, which declined to comment.

The subpoena to the state elections board requests documents from 2010 through August 2018, including voter registration applications, early voting application forms, and absentee ballot request forms.

Joshua Lawson, general counsel for the North Carolina state Board of Elections and Ethics Enforcement, estimated that it covered more than 15 million state documents.

The subpoena to the county boards requests “any and all poll books, e-poll books, voting records, and/or voter authorisation documents and executed official ballots” for a five-year period beginning August 30, 2013.

Mr Lawson said that this would cover in excess of 5 million county documents.

The subpoena request, he said, was “probably the largest on record in North Carolina within our agency.” He, like others, said he was puzzled by the investigation’s goals.

“With the scope so broad, what is the particular interest of immigration authorities?” he said.

The state elections board plans to consider the subpoenas in a telephone meeting on Friday and may discuss whether to resist them.