WASHINGTON (USA TODAY) -- A U.S. Justice Department review has identified at least 175 federal prisoners who must be released or resentenced because they have been locked up improperly.

The review, which followed a USA TODAY investigation, found that some of those prisoners shouldn't have been imprisoned because they hadn't committed a federal crime. Others received sentences vastly longer than the law allows.

The problems stem from a misunderstanding about which North Carolina state convictions were serious enough to outlaw gun possession or require extended prison sentences under federal law. The number of prisoners ultimately freed or given shorter sentences is likely to be higher than 175 because the examination by federal prosecutors was confined to the smallest of North Carolina's three U.S. court districts. Justice Department spokesman Wyn Hornbuckle said "many more" cases could be upended when all are reviewed.

USA TODAY's investigation in June identified 60 people imprisoned even though a U.S. appeals court said what they had done was not a federal crime. Still, Justice Department lawyers did almost nothing to notify prisoners - many unaware they were innocent - and asked federal judges to keep them locked up anyway. The department reversed that position in August.

Since then, federal judges have ordered the government to free at least 32 prisoners, and have taken 12 more off post-prison supervision, court records show. Some had served up to eight years before they were freed.

"That's a huge number," said University of San Francisco law professor Richard Leo. He said it is uncommon for any federal convictions to be overturned, let alone for so many involving a single issue.

The Justice Department's examination, completed in September but never made public, is the first estimate of just how big that number could be. Ripley Rand, the U.S. attorney in Greensboro, N.C., whose office conducted the review, said up to a third of gun cases his office prosecuted in recent years could be thrown out. So many prisoners have filed legal cases challenging their convictions that he has assigned three prosecutors to them full time.

Prosecutors in the state's two other districts did not conduct a similar case-by-case review, and don't yet know how many of their own cases are in jeopardy. Instead, officials said they are working with defense lawyers to identify cases that should be overturned.

One such prisoner is Travis Dixon, convicted in 2006 of illegally possessing a .44-caliber revolver. Court records show none of his prior state convictions was serious enough to make firearm possession a federal crime. In September, the Justice Department asked a U.S. court to overturn his conviction and let Dixon go.

But Dixon is still in federal prison in South Carolina while he waits for a judge to act on the request. "I'm just not understanding why they're taking this long," he said in an e-mail.

Another prisoner, Marion Howard, was freed only after he wrote a letter to the judge asking her to "please rule on my case before the holidays" so he could get home to see his family. The judge freed him on Dec. 5.

Federal law bans people from having a gun if they have previously been convicted of a crime that could have put them in prison for more than a year. In North Carolina, however, state law set the maximum punishment for a crime based in part on the criminal record of whoever committed it, meaning some people who committed crimes such as possessing cocaine faced sentences of more than a year, while those with shorter records face only a few months.

For years, federal courts there said that didn't matter. If someone with a long record could have gone to prison for more than a year, then all who had committed that crime are felons and cannot legally have a gun, the courts maintained. But last year, the 4th Circuit Court of Appeals said judges had been getting the law wrong, ruling that only people who could have faced more than a year in prison for their crimes qualify as felons.

The decision meant that low-level state convictions should not have been enough to outlaw gun possession or to justify extra-long prison sentences for people who went on to be convicted of federal crimes.

It will take at least a year to untangle all the cases like those, Rand said. His office's 20 criminal lawyers have been swamped by so many prisoners challenging their sentences that they have been forced to delay some other criminal prosecutions. "It's definitely been a huge burden," he said.