Healthcare insurance provider Premera Blue Cross revealed that it faced a cyber-attack that might have exposed medical records and financial information that belong to more than 10 million customers.

According to the health insurer, the attackers might have gained access to data involving clinical information, banking account numbers, birth dates, Social Security numbers and other data. Premera said that the attack took place last year.

As per Dave Kennedy, an expert in healthcare security who is chief executive of Trusted SEC LLC, it is the largest breach reported so far and it also includes medical information of patients. According to Premera, accounts of more than five million people were accessed and they are residents of Washington State. Customers involved are employees of Amazon Inc, Microsoft Corp and Starbucks Corp. The remaining ones are scattered across the United States.

No evidence has been found so far to show that member data was utilized improperly. According to a Starbucks spokesman, Premera informed them that they might have been affected by the attack.

Amazon's representatives have not replied to the requests for comment whereas Microsoft's representative refused to comment. On January 29, the Premera breach was discovered and it was the day when Anthem Inc also revealed a cyber-attack including data of more than 70 million members in Blue Cross Blue Shield plans across the country.

Spokesman of Premera, Eric Earling, said that the two attacks do not have any connection and his company separately identified the breach. FireEye Inc has been hired by Premera in order to investigate the matter. Premera said that it is also working with the FBI.

"Medical records paint a really personal picture of somebody's life and medical procedures. They allow you to perpetrate really in-depth medical fraud", said Kennedy.