Fitchburg, Mass. city officials reported that the Social Security numbers of 1,800 residents were compromised during a data breach that was discovered on April 14, but took place more than three years ago.

Fitchburg officials were unsure exactly how the breach occurred, reported to the Sentinel Enterprise. In a report filed by the city with the state Office of Consumer Affairs and Business Regulation on Aug. 28, 2013 data was either removed by a hacker or inadvertently removed from an employee's computer.

The data was encrypted, but the key was also released.

The city has informed local law enforcement and has started to notify the victims.