The Capital One episode is a reminder of the intricacy of the computer networks at large financial institutions, as well as their vulnerability. Over the last several years, companies including Equifax and Morgan Stanley have been attacked with various hacking methods.

In some cases, the hackers have taken advantage of weak passwords or sent fake emails loaded with malicious computer code that helped them get inside the network. In others, they have scanned for software that hasn’t been kept up-to-date with the latest security fixes. Some hacks took hours. Others took months.

“The very best hackers in the world are hacking these banks, and it’s a full-fledged arms race,” said Tom Kellermann, the chief cybersecurity officer at Carbon Black, a security software maker.

It is unclear whether any sort of insider information helped Ms. Thompson break into the Capital One network, as prosecutors allege. Though her online résumé indicated that she had a wide range of programming skills, it did not appear that the breach of the bank’s computer systems was particularly sophisticated.

Three years ago, Ms. Thompson worked for Amazon Web Services, the cloud computing service that hosted Capital One’s data. But she left the company long before the breach. Amazon manages the guts of Capital One’s network — the servers and networking technology that hold it together. The software Ms. Thompson is alleged to have targeted would have been managed by the bank itself.

Ms. Thompson used a gap in Capital One’s firewall software — a security system that acts like a digital gate — to gain security credentials, according to court documents. Then she gained access to customer records Capital One had stored on Amazon’s cloud service.

Representatives of Capital One refused to answer questions about whether Ms. Thompson had hacked into its systems or simply climbed through a window that had accidentally been left open.