The 23 boxes included a file titled "phones/wiped" with the names of eight state employees, prosecutors said.
"Only within the past few weeks did the People learn of a trove of documents and other materials that should have been, but was not, provided to it months or even years ago," the attorney general's office said in a court filing.
It's unclear what connection, if any, the boxes have to former health chief Nick Lyon, who is charged with involuntary manslaughter in an outbreak of Legionnaires' disease. He's accused of failing to timely warn the public about the disease while Flint was using water from the Flint River in 2014-15 . The water wasn't properly treated, which caused lead to leach from old pipes, among other problems.
Lyon has been ordered to trial , but Genesee County Judge Joseph Farah is considering an appeal. He planned to release a decision no later than May 17, more than three months after hearing arguments.
"We were in full overdrive working to get this done," the judge told The Associated Press.
But prosecutors now want him to suspend the case for six months. Farah said he'll hold a hearing on May 3.
"We will vigorously oppose it. This is another stall tactic by the prosecution," said Lyon's attorney, Chip Chamberlain.
Prosecutors didn't suddenly stumble upon the documents and computer hard drives. They said they were informed in February by Peter Manning, a division chief in the attorney general's office, that the boxes "were languishing in the basement of a state-owned building."
Other attorneys in the office were aware of the records because they were defending state officials in civil lawsuits related to the Flint water scandal, Assistant Attorney General Daniel Ping said.
They indicated that the records were duplicates of what already had been given to prosecutors, but investigators said that wasn't true, Ping wrote.
The criminal investigation was led by a special prosecutor, Todd Flood, until earlier this year when Dana Nessel, the new attorney general, brought the cases into her office and formed a new team. Flood charged 15 people; so far seven have pleaded no contest to misdemeanors.
This week was the fifth anniversary of Flint's switch to the Flint River. The city used the river for 18 months until fall 2015.
Follow Ed White at http://twitter.com/edwhiteap