The Flint Water Plant water tower in Flint, Mich., on March 21, 2016. Prosecutors dropped all criminal charges Thursday against eight people in the Flint water scandal and pledged to start the investigation from scratch. (Carlos Osorio/AP)

The office of Michigan’s attorney general dismissed every pending criminal case related to the water crisis in Flint on Thursday, effectively restarting an investigation that began three years earlier and grew to encompass charges against multiple state and local officials.

Authorities described the decision to dismiss without prejudice charges against eight people as a necessary corrective to an inquiry they said had been flawed until now. They also pledged a “vigorous pursuit of justice” and said their ongoing investigation had uncovered more information and evidence related to the crisis.

“We are now in the best possible position to find the answers the citizens of Flint deserve and hold all responsible parties accountable,” Fadwa A. Hammoud, who was named Michigan’s solicitor general this year and tapped to lead the Flint criminal cases, said in a statement issued with Wayne County Prosecutor Kym L. Worthy.

“Our team has already identified additional individuals of interest and new information relevant to the Flint Water Crisis,” they added. “These investigative leads will be aggressively pursued.”

The move Thursday comes as Flint residents and others have sought for years to see people held accountable for pumping lead-tainted water into residents’ homes, prompting a public-health crisis.

Michigan Attorney General Dana Nessel, a Democrat elected last year, said in a statement that she trusted Hammoud and Worthy’s decisions to dismiss the charges.

“I want to remind the people of Flint that justice delayed is not always justice denied and a fearless and dedicated team of career prosecutors and investigators are hard at work to ensure those who harmed you are held accountable,” Nessel said.

For decades, the once-thriving industrial city of Flint had paid Detroit to have its water piped in from Lake Huron, with anti-corrosion chemicals added along the way. But in early 2014, with the cash-strapped city under the control of a state-appointed emergency manager appointed by Republican Gov. Rick Snyder, came the decision to switch to Flint River water to save money.

When the switch took place in April 2014, state officials failed to ensure that proper “corrosion-control” treatments — which would have cost little to implement — were added to the new water supply. That oversight allowed rust, iron and lead to leach from aging pipes and wind up in residents’ homes. Thousands of children were exposed to high levels of lead, which has to the potential to cause long-term physical damage and mental impairment.

Residents began complaining almost immediately after the switch to Flint River water. Their tap water was discolored and foul-smelling, they said, and skin rashes appeared after bathing in it. Those complaints led to little action. For months, public officials insisted that the water was safe and dismissed the growing concerns from Flint residents.

Mona Hanna-Attisha, the Flint pediatrician whose research in late 2015 first documented dangerously high lead levels in children’s blood, once said that residents in the largely poor and minority city “were forgotten and neglected by every agency in the country that was supposed to protect them.”

After the damage became clear, state and federal officials sprang into action The EPA eventually used its emergency powers to demand action by the state and city. Its regional leader resigned. So did the state’s water quality director. The National Guard handed out bottled water, and water filters were distributed. State and federal investigations got underway. Amid calls to resign, Snyder apologized and told Flint residents in one State of the State address that “government failed you at the federal, state and local level.”

Despite all that, the crisis prompted widespread criticism and devastated local residents’ trust in government. Many residents said that trust might never be restored.

In their statement, Hammoud and Worthy described their decision to dismiss the charges as “voluntary.” They also left open the possibility that some of those who had their charges dropped could be charged again. Nessel’s office said Hammoud and Worthy would not respond to media questions until after a community meeting in Flint planned for June 28.

Hammoud and Worthy said once they took over the case, their team of prosecutors and investigators “had immediate and grave concerns about the investigative approach and legal theories embraced by” by the authorities previously leading the investigation.

Hammoud and Worthy said their evaluation of the prosecution until this year determined that “all available evidence was not pursued” and that private firms representing Michigan agencies were able to help decide what to turn over. Their team tried to “salvage whatever progress had been made,” the attorneys said, but eventually determined that they could not “build on a flawed foundation” in the case.

Todd Flood, the prosecutor appointed as special counsel to lead the Flint investigation in 2016, defended his team’s efforts and said they stood by their work.

“The focus of myself and our team remains as it always has been: Justice for the people of Flint,” Flood said in a prepared statement he read to The Washington Post. He added: “I am confident our efforts on behalf of the thousands of victims met the highest professional and ethical standards.”

Flood said his team had obtained cooperation agreements and guilty pleas, interviewed hundreds of witnesses and scoured millions of pages of evidence. He also said they wish Hammoud and Worthy success as they proceed with the inquiry.

“We stand ready to assist in the investigation as it moves forward to deliver justice for Flint,” he said.

Bill Schuette, a Republican who preceded Nessel in office and appointed Flood, also spoke up for the years-long investigation on Thursday, writing on Twitter: “We had an experienced, aggressive and hard-driving team. Everything we did was for the people of Flint.”

Hanna-Attisha, the Flint pediatrician, wrote in an email that she hopes that more answers are uncovered through the ongoing investigation.

“Accountability and restorative justice are critical to healing,” she said. “Today’s news that prosecutors dismissed the Flint water crisis criminal cases in an effort to restart the investigations is painful. . . . My hope is that this development will shine new light in efforts to uncover the truth. There are so many unanswered questions.”

Those who had charges dismissed Thursday included Nick Lyon, former director of Michigan Department of Health and Human Services, and Eden Wells, the state’s chief medical executive.

“Needless to say, we’re elated,” said Chip Chamberlain, lawyer for Michigan’s former health director Nick Lyon, said in an interview Thursday. “We thought this was going to be the ultimate outcome anyway through the courts. But to have it come sooner is far better.”

Lyon was charged with involuntary manslaughter in connection with the crisis and had spent years in court battling accusations that he failed to quickly inform the public about an outbreak of Legionnaires’ disease that coincided with the water problems.

Chamberlain acknowledged that the state could technically refile charges against Lyon, but he expressed confidence that won’t happen, arguing that the original charges were politically motivated.

“We’re not the least bit worried,” he said. “Any fair investigation is going to reveal that Nick did not commit a crime. If you take politics out of it, there isn’t any crime.”

Attorneys for Wells echoed that sentiment about the ongoing investigation.

“We understand that there will be further investigation, but do not expect it to justify any further prosecution relative to her,” Steven Tramontin and Jerold Lax, Wells’s attorneys, said in a statement. “The Solicitor General is now free to pursue her investigation while Dr. Wells is free to continue her distinguished career in public health.”