Michael Nagle/Bloomberg | Getty Images

A software engineer accused of hacking into Capital One and stealing the data of more than 100 million people tweeted about euthanizing her cat, Millie, and seeking mental help, authorities said. The FBI arrested 33-year-old Paige Thompson of Seattle on Monday, saying she is responsible for the Capital One digital break-in. Thompson faces federal charges of computer fraud and "abuse for an intrusion on the stored data." The FBI said it tracked down her with help from Capital One, which received a tip about the stolen data in an email. Federal agents were able to link her identity to social media and user accounts, known online by the alias "erratic." She is believed to have acted alone, investigators said. In other major hacks, Equifax and Marriott were attacked from the outside by criminals with a nation-state connection. Thompson worked in the Seattle area as a technology company software engineer, the Justice Department said. It said she intruded "into servers rented or contracted" by Capital One, as well as "from a company that provides cloud computing services."

The DOJ did not mention the name of the company, but it said Thompson worked at a cloud software company from 2015 to 2016. An Amazon spokesperson confirmed that Thompson worked for Amazon Web Services, but left in 2016. "AWS was not compromised in any way and functioned as designed," Amazon said in a statement to CNBC. "The perpetrator gained access through a misconfiguration of the web application and not the underlying cloud-based infrastructure. ... This type of vulnerability is not specific to the cloud."

Capital One gets a tip

The financial institution received an anonymous tip on July 17 from a person who said in an email that "there appears to be some leaked s3 data of yours in someone's github ... let me know if you want help tracking them down," the a screenshot showed. The DOJ said the GitHub file "contained the IP address for a specific server" of Capital One, which had "a firewall misconfiguration." That "permitted commands to reach and be executed by that service, which enabled access to folders or buckets of data in Capital One's storage space the Cloud Computing Company," investigators said. Thompson allegedly took data that was primarily "related to credit card applications," the complaint said, including about 120,000 Social Security numbers and about 77,000 bank account numbers.

FBI tracks Thompson through social media accounts