Michigan prosecutors pledge to start probe into one of the worst man-made environmental disasters in US history.

Prosecutors in the US state of Michigan have dropped all criminal charges against eight people in the Flint water scandal, pledging to start the investigation from scratch.

The move came on Thursday, three years after the investigation began, putting no one behind bars for one of the worst man-man environmental disasters in US history.

The defendants included 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 2014-2015 when Flint was drawing improperly treated water from the Flint River.

That led to residents’ water being tainted with lead.

Seven of 15 defendants have pleaded no contest to misdemeanours. Their records will eventually be scrubbed clean.

Expanded scope

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.

“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.

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

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 percent 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 vapour, typically from cooling systems.

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.