Image copyright Reuters Image caption Michigan's attorney general says four more people have been charged in an investigation of Flint's lead-tainted water system

Four more city workers have been charged in the investigation into the lead-tainted water crisis in Flint, Michigan, state prosecutors said.

Former state-appointed emergency managers Darnell Earley and Gerald Ambrose were charged for failing to protect residents from the toxic water.

Two city employees were also charged with felony counts related to the case.

Flint's drinking water became contaminated with lead in 2014 after the city changed its water supply.

Attorney General Bill Schuette charged Mr Earley and Mr Ambrose with false pretences, conspiracy to commit false pretences, misconduct in office and wilful neglect of duty in office.

He also charged former city employees Howard Croft, a public works superintendent, and Daugherty Johnson, a utilities manager, with conspiracy to operate a plant that was not equipped to properly treat water from the Flint River.

"The tragedy that we know of as the Flint water crisis did not occur by accident," Mr Schuette said.

"Flint was a casualty of arrogance, disdain and failure of management, an absence of accountability."

Mr Johnson's attorney, Edwar Zeineh, told the AP news agency his client will plead not guilty.

The others and their lawyers could not immediately be reached for comment.

The water crisis made national headlines after tests found high amounts of lead in blood samples taken from children in the majority African-American city.

Image copyright AP Image caption Although the water has improved, many Flint residents still use bottled water for drinking

The contamination can be traced back to April 2014, when Mr Earley, the emergency manager at the time, decided to change the city's water source from Lake Huron to the Flint River to save money.

The acidic water of Flint River corroded the city's pipes, which leached lead into the water.

Flint switched back to the previous water system in October 2015 and federal regulators say that filter tap water is now safe to drink, but still recommend bottled water for young children and pregnant women.

So far 13 people have been charged in connection to the investigation into Flint's water supply and an outbreak of Legionnaires' disease.

The state brought charges against two state regulators and a city employee for official misconduct in April.

In July, prosecutors charged six health and environmental workers for concealing data that showed elevated levels of lead in children's blood was tied to the water supply.

Mr Early and Mr Ambrose face up to 46 years in prison while Mr Croft and Mr Johnson could face up to 40 years behind bars.

Mr Schuette's announcement comes a day after a report found nearly 3,000 areas with recently recorded lead poisoning rates twice as much as those in Flint during the height of the city's water crisis.

More than 1,100 of those communities had a rate of elevated blood tests at least four times higher, according to the Reuters report.