Education officials say they will retest all New York City public school buildings for possible unsafe levels of lead.
This time the tests will be performed under new state regulations that discourage flushing of water the night before testing to clear the outlets of sediment. That practice has recently drawn criticism from some experts.
The testing is expected to conclude by June 2017.
According to the Daily News, elevated lead levels were detected at 509 public school buildings in water samples taken in March, April and May 2016. But officials say the levels were not harmful and that any cases were corrected.
The city's Department of Education announced the new effort in a letter sent home to parents on Monday.