Authorities in the US state of Michigan filed criminal charges Friday against six current and former state officials over lead water contamination in the city of Flint, signaling that their probe is expanding.

"Many things went tragically wrong in Flint. Some failed to act," Michigan State Attorney General Bill Schuette told a news conference as he announced the new charges.

"Some intentionally altered figures, and covered up."

More than 8,000 children are believed to have consumed lead-tainted water in the hardscrabble northern city, which has become a focus of the 2016 White House race.

Until now, three state workers had been criminally charged over the health crisis, which occurred in the aftermath of government officials' cost-saving effort in 2014 to switch the city's source of drinking water.

One took a plea agreement to cooperate in the probe.

Chief investigator Andrew Arena said his team was "starting to work our way up" in the areas of state government they were investigating, and expanding the probe "into other departments."

"This at the end will be the largest criminal investigation in the history of the state of Michigan," he said.

On Friday authorities charged another six current and former state employees for covering up evidence of water contamination. They face various felony and misdemeanor counts, including conspiracy, misconduct in office and tampering with evidence.

Experts say the new source of water proved more corrosive on aging pipes and caused lead contamination that exposed thousands of children to the toxin.

"In essence, these individuals concealed the truth. They were criminally wrong to do so," Schuette said.

The highest ranked official charged is Liane Shekter Smith, who was fired in February from her post as chief of Michigan's Department of Environmental Quality.

"Shekter Smith ignored reports that the (water treatment) plant was out of compliance, lied that the water plant was certified, and deliberately misled her superiors," Schuette said.

She faces a felony charge of misconduct in office and a misdemeanor charge of willful neglect of duty.

The others charged are: Nancy Peeler and Robert Scott, who are current employees in the Michigan Department of Health and Human Services; Corrine Miller, a former MDHHS employee; and Patrick Cook and Adam Rosenthal, current employees of the state's Department of Environmental Quality.

Todd Flood, special counsel for the Flint probe, said he was in talks with Michigan Governor Rick Snyder's office in connection with the investigation and that talks had been "constructive."

The damage to the city's water pipes may be long-lasting, if not permanent. Residents must use filters to make their water drinkable.

Last month officials also filed a lawsuit accusing a French company and a Texas firm of negligence and fraud, for their roles in the Flint water crisis.