NEW YORK — After a presidential campaign scarred by Russian meddling, local, state. and federal agencies have conducted few digital forensic investigations to assess the possible effect of hacking on the vote, according to nearly two dozen national security and state officials and election technology specialists.

Voting systems in at least 21 states were targeted by Russian hackers, security officials have found.

One prominent example of Election Day problems occured in North Carolina, where calls started pouring in from hundreds of irate voters just after 7 a.m. on Nov. 7.

Dozens of people said they were told they were ineligible to vote and were turned away at the polls, even when they displayed current registration cards. Others were sent from one polling place to another, only to be rejected.

Scores of voters were incorrectly told they had cast ballots days earlier. In one precinct, voting was halted for two hours.

Susan Greenhalgh, a troubleshooter at a nonpartisan election monitoring group, was alarmed. Most of the complaints came from Durham, a blue-leaning county in a swing state.

The problems involved electronic poll books — tablets and laptops loaded with check-in software that have increasingly replaced the binders of paper used to verify voters’ identities and registration status.

She knew that the company that provided Durham’s software, VR Systems, had been penetrated by Russian hackers months before. “It felt like tampering, or some kind of cyberattack,’’ Greenhalgh said about the voting troubles in Durham.

There are plenty of other reasons for such breakdowns — local officials blamed human error and software malfunctions — and no clear-cut evidence of digital sabotage has emerged, much less a Russian role in it.

Despite the disruptions, a record number of votes were cast in Durham, following a pattern there of overwhelming support for Democratic presidential candidates, this time Hillary Clinton.

But months later, for Greenhalgh, other election security experts and some state officials, questions still linger about what happened that day in Durham as well as other counties in North Carolina, Virginia, Georgia, and Arizona.

The assaults on the vast back-end election apparatus — voter-registration operations, state and local election databases, e-poll books, and other equipment — have received far less attention than other aspects of the Russian interference, such as the hacking of Democratic e-mails and spreading of false or damaging information about Clinton.

Yet the hacking of electoral systems was more extensive than previously disclosed, The New York Times found.

Beyond VR Systems, hackers breached at least two other providers of critical election services well before the 2016 voting, said current and former intelligence officials, speaking on condition of anonymity because the information is classified. The officials would not disclose the names of the companies.

Intelligence officials in January reassured Americans that there was no indication that Russian hackers had altered the vote count on Election Day, the bottom-line outcome. But the assurances stopped there.

Government officials said that they intentionally did not address the security of the back-end election systems, whose disruption could prevent voters from even casting ballots.

That’s partly because states control elections; they have fewer resources than the federal government but have long been loath to allow even cursory federal intrusions into the voting process.

That, along with legal constraints on intelligence agencies’ involvement in domestic issues, has hobbled any broad examination of Russian efforts to compromise US election systems.

Those attempts include combing through voter databases, scanning for vulnerabilities or seeking to alter data, which have been identified in multiple states.

Current congressional inquiries and the special counsel’s Russia investigation have not focused on the matter.

“We don’t know if any of the problems were an accident, or the random problems you get with computer systems, or whether it was a local hacker, or actual malfeasance by a sovereign nation-state,’’ said Michael Daniel, who served as the cybersecurity coordinator in the Obama White House.