“Upon assuming responsibility of this case, our team of career prosecutors and investigators had immediate and grave concerns about the investigative approach and legal theories,” said the two lead prosecutors, Fadwa Hammoud and Kym Worthy, in a statement. They said they would meet with Flint residents later this month and noted they were “not precluded from refiling charges against the defendants” or adding new charges and defendants.

[Read more about the seizure of the former governor’s devices.]

Flint’s mayor, Karen Weaver, said she took the prosecutors at their word and hoped they would follow through with new charges. She said that there was some confusion and frustration in her city about the decision to drop charges, but that she believed it could ultimately be a positive.

“It is frustrating, but I’d rather be frustrated at this end and know that they’re going to do a deep dive into what happened,” Ms. Weaver said in an interview. She added: “I think this way, they may have the evidence they need to be able to hold them accountable and throw away the key.”

Efforts to speak with Mr. Flood, the former lead prosecutor, and Mr. Schuette, the former attorney general, on Thursday were not immediately successful. Mr. Schuette defended his team’s work on Twitter. “We had an experienced, aggressive and hard-driving team,” he wrote. “Everything we did was for the people of Flint.”

Among the officials whose charges were dropped: the former director of the Michigan Department of Health and Human Services, a state epidemiologist, a former Flint public works director, and emergency managers who had been appointed by Governor Snyder to oversee the city. The decision to drop charges did not affect seven officials who had already accepted deals with prosecutors and entered no-contest pleas, but who had not yet been formally convicted by a judge.

James White, a lawyer for Howard Croft, the former Flint public works director, who was charged with involuntary manslaughter and has denied wrongdoing, said the attorney general’s decision validated his concerns about the investigation. Mr. White said his client, who was first charged in 2016, was “carefully, cautiously elated” about the news.

“I give a lot of the credit to the attorney general for having the courage to do this,” Mr. White said.