The manager of the public water laboratory in Saipan Island has been charged with making up water quality tests.
A federal indictment alleges Pedro 'Pete' Babauta falsified monthly water quality tests six times.
A test would indicate water is clean, even if it tested positive for bacterial contamination.
The bacteria is not thought to be harmful to humans, but coliform bacteria could indicate the presence of more dangerous bacteria.
Most people in Saipan drink bottled water, but tap water is widely used for bathing, washing dishes and cleaning vegetable.
Patrick Smith, who's acting as a prosecutor for the US attorney in the case, said he didn't know what Mr Babauta's motivation, in allegedly faking the tests, could have been.