Swedish Medical Center is alerting nearly 20,000 current and former employees that their personal information, including Social Security numbers, was accidentally accessible on the Internet for nine weeks.

There is no evidence any of the information has been used for identity theft or other illicit purposes, Swedish said Wednesday. Joanne Suffis, vice president of Human Resources, said the incident was highly unusual. Swedish is providing affected people with free access to an identity-theft service.

The data breach affected some, but not all, of the hospital's current or former employees who worked there at some point between 1941 and 2006.

The breach occurred in April, when a Swedish employee working at home made changes to his home network that accidentally allowed others to access information on his computer through an Internet search. Swedish policy prohibits keeping confidential information on home computers.

Suffis said Swedish will be doing additional staff education and beefing up technical security.

Carol Ostrom: 206-464-2249 or costrom@seattletimes.com

Information in this article, originally published July 20, 2011, was corrected August 10, 2011. Due to incorrect information provided by Swedish Medical Center, a previous version stated that personal information had been made accessible on the internet about some current and former employees who worked at Swedish during 1994, 1995, 2002, 2003, 2004 or 2006. The hospital has revised those dates, saying the data breach affected some, but not all, of the hospital's current or former employees who worked there at some point between 1941 and 2006.