Prosecutors stunningly dropped all criminal charges Thursday against eight people in the Flint water scandal and pledged to start the investigation from scratch. The defendants include Michigan's former health director, Nick Lyon, who was charged with involuntary manslaughter.

He was accused of failing to timely alert the public about an outbreak of Legionnaires' disease that occurred in 2014-15 when Flint was drawing improperly treated water from the Flint River. That led to residents' water being tainted with lead, causing one of the worst man-made environmental disasters in U.S. history.

Michigan Solicitor General Fadwa Hammoud, who took control of the investigation in January after the election of a new attorney general, said "all available evidence was not pursued" by the previous team of prosecutors.

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"This week, we completed the transfer into our possession millions of documents and hundreds of new electronic devices, significantly expanding the scope of our investigation," Hammoud and Wayne County prosecutor Kym Worthy said in a statement.

"Our team's efforts have produced the most comprehensive body of evidence to date related to the Flint water crisis. We are now in the best possible position to find the answers the citizens of Flint deserve and hold all responsible parties accountable," they said.

Nick Lyon testifies on April 13, 2016, on Capitol Hill in Washington, D.C. Getty

They said it is possible that Lyon and others could be charged again.

Chip Chamberlain, Lyon's lawyer, said they feel "fantastic and vindicated" by the dismissal of charges Thursday. He said he's confident a new investigation "will yield no evidence of any criminal wrongdoing."

While waiting for a new pipeline to bring water from Lake Huron, the majority-black city of 100,000 pulled water from the Flint River without treating it to reduce corrosive effects on old pipes. Lead infected the distribution system in Flint, where 41% of residents are classified by the government as living in poverty.

Due to poor finances, Flint was being run by financial managers appointed by the governor. The uproar over water quality reached a peak by fall 2015 when a doctor reported high levels of lead in children, which can cause brain damage.

Some experts also have linked the water to Legionnaires' disease, a type of pneumonia caused by bacteria that thrive in warm water and infect the lungs. People can get sick if they inhale mist or vapor, typically from cooling systems.

Legionella bacteria is especially dangerous for people with weakened immune systems. The state says there were 90 cases reported to Genesee County Health Department in 2014-15, including 12 deaths. More than half of the people had a common thread: They spent time at McLaren Hospital, which was on the Flint water system.

In August 2016, Troy Kidd's mother Debbie died of Legionnaires' disease. "Did she die because someone didn't want to say, 'Hey, the water might be bad?' That's my questions," Kidd told CBS News at the time.

The outbreak was announced by Gov. Rick Snyder and Lyon in January 2016, although Lyon concedes that he knew that cases were being reported a year earlier. "He had the chance to save lives," special prosecutor Todd Flood told a court hearing in 2018.

The city's water no longer comes from the river and has significantly improved, but some residents are so distrustful that they continue to use bottled water.

Three years after the investigation began, no one is behind bars. Seven of 15 defendants have pleaded no contest to misdemeanors. Their records will eventually be scrubbed clean. Charges were pending against eight people, including former Michigan chief medical executive Eden Wells and two men who were state-appointed emergency managers in Flint.

Like Lyon, Wells was charged with involuntary manslaughter.

"We understand that there will be further investigation, but do not expect it to justify any further prosecution relative to her," Wells' attorneys, Steven Tramontin and Jerold Lax, said.

Through February, Flood's team had spent $8.2 million, records show. State government separately has spent millions of dollars to provide defense lawyers to public employees charged in the case.

Hammoud dismissed Flood in April and put together a new team. He didn't immediately respond to a request for comment on the dismissal of charges Thursday.