As many as 17,000 students in Newark, New Jersey schools could be tested for lead in their blood after findings showed elevated levels of the toxin have been in water in schools since at least 2012, city health officials said.

Voluntary lead testing began on Thursday in the state's largest school district after 30 schools were found to have high levels of lead in the water fountains last week. The school district has about 35,000 students.

Health officials said the testing started with pupils at two early childhood centers, which were among schools where water fountains were shut off on March 9 after recent testing found lead levels exceeded the federal safety limit.

Officials earlier this week acknowledged the problem has plagued the district since 2012.

Lead has not been found in the water supply of the city of Newark, located 11 miles west of New York City, the state Department of Environmental Protection (DEP) confirmed.

RELATED: Elevated levels of lead found in water throughout New Jersey school system

Still, the issue has brought comparisons to the crisis in Flint, Michigan. At a Congressional hearing on Thursday, Michigan Governor Rick Snyder testified that the lead contamination in water resulted from the cumulative failures of local, state and federal governments.

In Newark, the state DEP plans to test water at all 67 Newark public schools beginning on Saturday, starting with 13 charter schools and non-traditional school buildings, such as a athletic facilities, which were not tested this school year. It will then retest the 30 school buildings where lead levels above the federal limit of 15 parts per billion were found.

Nearly all the water taps where the highest levels were found are typically not used for drinking or food preparation, the school district said.

Lead issues date back as far as 2004, when remedial action was taken. Data collected by the district's independent laboratory showed 12 percent of 2,067 water quality samples - collected from 2012 to 2015 - had lead levels above the federal limit requiring action.

Christopher Cerf, the new state-appointed school superintendent, declined to say why the district did not previously make public that information.

"Without intending to criticize any of my three predecessors, when I learned of the 2015 test results, I decided to address the situation differently," he said in a statement on Wednesday. "Within an hour, I had notified state and city officials and directed staff to connect with the State Department of Environmental Protection."