The lone hacker who allegedly accessed data from over 100 million Capital One customers had also also breached dozens of other companies and institutions.

Paige Thompson, who used the online name 'erratic', was arrested last month for committing one of the biggest data breaches in recent memory.

She stole fourteen years' worth of customer data — including social security and banking details — and posted the evidence on GitHub.

Analysis of servers seized during the search of Thompson’s home found terabytes of stolen data from over 30 companies other than Capital One.

She said she never copied, sold, or distributed any of the data, but apparently admitted to the Capital One breach upon arrest.

Thompson has been unemployed since 2016 when she worked as an engineer for Amazon Web Services — which was used by Capital One for its data storage.

Capital One said it expected Thompson’s intrusion to cost more than US$100 million this year alone.

Its stock price dropped more than 10 per cent since the breach was announced in July.

A class action lawsuit has been filed against the financial institution for “failure to exercise reasonable care in securing and safeguarding consumers’ sensitive personal information”.

The Wall Street Journal recently reported that Capital One’s IT cyber security team had seen a significant turnover two years leading up to the breach and that it had delayed updating its system to ward off attacks.

US attorneys are pushing for Thompson to remain in custody pending her trial, saying she posed a significant threat to the community.

Police had been called to Thompson’s house earlier this year after she threatened to shoot up the offices of a Californian tech company.

Thompson’s roommate and landlord, 66-year-old Park Quan, was in possession of 14 firearms, including AR-15– and AK-47–style assault rifles when the hacker was arrested.

While enacting a search warrant on Thompson’s home, FBI agents spotted what court documents called “an arsenal of weapons, ammunition, and explosive material” in Quan’s bedroom.

US attorneys said Thompson's close proximity to weapons, and people who own them, was reason enough to keep her detained.

"The fact that all of these weapons were recovered in the bedroom adjacent to Thompson, most of them readily accessible to her, is obviously of concern, given Thompson's recurrent threats to commit violence against herself and others," the court documents said.

“In today’s America, it is easy enough to obtain firearms, and there is every reason to be concerned that Thompson, who repeatedly has threatened to kill, would obtain the means to carry out her threats.”