Hartford schools postponed the first day back to school today due to a ransomware virus attack, officials say.

The announcement was made by Hartford Public Schools in a letter to parents Tuesday that read in part, "We have been informed by Metro Hartford Information Services, our City of Hartford shared services team that manages our network infrastructure, that the ransomware virus caused an outage of critical systems and the restoration of those systems are not complete."

Officials say the virus has infected the system that communicates city transportation routes to the bus company, and it is preventing the district’s ability to operate schools.

Officials did not say when schools would be back open.