Millions were believed to have been hit after SpamHaus, a spam-fighting group, was targeted by a web filtering firm they had black listed.
It was one of the largest attacks of its kind, impacting popular websites like Netflix. Now experts worry that it could affect banking and email systems.
Police are now investigating the attacks, which are being called "unprecedented." It came at the same time as the British government launched an initiative to help businesses and law enforcement agencies share information on cyber attacks.
"These things are essentially like nuclear bombs," CEO of Internet security firm CloudFlare, Matthew Prince, said.
SpamHaus is located in London and Geneva, and is a nonprofit whose goal is to help email providers get rid of spam.
It does so by maintaining a list of databases that are known to be malicious and blocking them.
Dutch firm CyberBunker was recently added to their list.
A spokesperson for SpamHaus said that the firm sought revenge for being added to their blacklist by retaliating and performing a "denial of service attack."
The attacks happen by making a certain network unavailable to its users. They are believed to do this by overloading servers with coordinated requests to access it.
Patrick Gilmore, a chief architect at Akamai Networks, said that CyberBunker is just mad they were caught.
"These guys are just mad. To be frank, they got caught. They think they should be allowed to spam," he said.
Gilmore said the attacks were launched by botnets, or a collection of computers that concentrate data streams that can sometimes be larger than the Internet connections of whole countries.
He compared the method to "using a machine gun to spray an entire crowd when the intent is to kill one person."