DETROIT (Reuters) - Two Michigan state officials criminally charged in an investigation into dangerous lead levels in the city of Flint’s drinking water pleaded not guilty, a court spokeswoman said on Thursday.

Stephen Busch and Michael Prysby, both Michigan Department of Environmental Quality (MDEQ) officials, entered not guilty pleas on Wednesday, Dena Altheide, a court administrator with the 67th District Court of Genesee County in Flint, said in an email.

Busch, 40, and Prysby, 53, were charged on Wednesday with five and six counts, respectively, including misconduct in office, tampering with evidence and violation of the Michigan Safe Drinking Water Act.

However, official misconduct over Flint may be hard to prove, according to lawyers familiar with Michigan criminal law.

Attorneys for both men could not be reached to comment. A probable cause hearing for both men is scheduled for May 4.

Both men were released on their own personal recognizance, court documents show.

Flint city employee Michael Glasgow, 40, has not been arraigned and an attorney for him is not listed in court documents, Altheide said. He was charged with tampering with evidence by falsifying reports to state environmental officials, and willful neglect of duty.

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All three men are on unpaid leave, officials said.

If convicted, Glasgow faces up to five years in prison and $6,000 in fines, while Busch faces up to 15 years and $35,000 in fines, and Prysby faces up to 20 years and $45,000 in fines, according to court documents.

Flint was under control of a state-appointed emergency manager in 2014 when it switched its source of water from Detroit’s municipal system to the Flint River to save money.

The river water was more corrosive than the Detroit system’s and caused more lead to leach from its aging pipes. Lead can be toxic and children are especially vulnerable.

The crisis has prompted lawsuits by parents who say their children are showing dangerously high blood levels of lead. The city switched it back in October.

Michigan Attorney General Bill Schuette said Wednesday more charges are likely to be filed and the three men were cooperating with investigators.

Some critics have called for higher-ranking state officials, including Governor Rick Snyder, to be charged. Snyder said Wednesday he did not believe he had done anything criminally wrong.

U.S. Representative Dan Kildee, a Michigan Democrat, told CNN Thursday he hoped others would be charged.

“We want to make sure that the price is not just paid by a few people who are basically at the bottom of the chain, but it also applies to the people who created the culture that made it OK for these folks to manipulate test samples,” he said.