The North East King County Regional Public Safety Communication Agency, or NORCOM, announced Monday it was working with local and federal agencies to investigate the hack.
In all, an estimated 6,000 medical response records from Duval Fire District 45, Skykomish Fire Department and Snoqualmie Pass Fire & Rescue were left vulnerable by the breach.
The records include names, addresses, dates of birth, nature of emergency call and initial medical condition, according to NORCOM.
The breached files also contained personnel data for 231 full-time and volunteer firefighters who work or have worked for the three agencies.
"Immediately when the breach was discovered, NORCOM took swift actions to identify and analyze the depth of the breach; we are working with the Bellevue Police Department and with the United States Secret Service's Electronic Crimes Task Force on this investigation," said Tony Orr, NORCOM's executive director, in a Monday evening news release.
Orr went on to say NORCOM is now in the process of notifying the people who were affected by the breach.
NORCOM said the sever was hacked in late December. The server, which is not connected to the 911 computer system, is not longer in service.