WASHINGTON - The bad news piled up quickly for Carol Keller late last year. She was informed in December that her personal and medical information had been stolen nearly four months earlier when a Pentagon contractor left 25 computer tapes in the back seat of a Honda Civic in Texas. That explained the fraudulent purchases from her debit account, the Revere woman contends.

Keller, who is married to a disabled Air Force veteran and relies on the Pentagon-run health insurance program called TRICARE, is among 70,000 military personnel, retirees, and their families across New England who are grappling with the potential fallout of one of the largest-ever breaches of medical data. Nationally, as many as 4.7 million people may be vulnerable.

Keller insists the theft and unauthorized purchases are related and has joined nearly a dozen others in a class-action lawsuit seeking unspecified damages. Frustrated lawmakers and privacy specialists say the case spotlights what they contend is an ill-designed health system, in which the Pentagon relies on contractors and outdated computer storage technologies to house and transport personal information.

As a result of the outdated system, they say, those who risk their lives for the nation face undue risk of invasion of privacy and identity theft, and national security could be compromised.

“The bottom line is that people in charge of safeguarding our service members’ personal data need to transition from the 20th century to the era of iPads,’’ said Representative Edward J. Markey, who is demanding more answers from the Pentagon on its medical privacy policies. “TRICARE had given me no assurance that it is moving toward such a modern system.’’

Many of the questions concerning standards and technology center on the Pentagon’s use of contractor Science Applications International Corp. The contractor alerted Keller to the September breach weeks later - in a letter titled “urgent.’’