A North Carolina county says hackers have tried new attacks on its computer systems after it refused to pay them a ransom for frozen data.

Mecklenburg County manager Dena Diorio sent an email Thursday to county employees advising them that hackers were targeting the county primarily through fraudulent email attachments. No further damage to the systems was reported.

Diorio said the county was disabling employees’ ability to open attachments generated through Dropbox and Google Docs. She advised staff to generally limit its use of attachments overall and try to conduct business by phone or in person when possible.