The U.S.-based cloud company behind this week's major internet outage said on Wednesday the incident was caused by a bug in its software.
Tuesday's outage caused thousands of websites, including Amazon, PayPal and the New York Times, to go offline for up to an hour.
The UK government's website was also down.
Fastly said the bug was in a software update which was shipped to customers on May 12.
It was only when one unidentified customer carried out settings changes that the problem was triggered, causing 85% of the network to return errors.
In a statement, Fastly said:
"This outage was broad and severe, and we're truly sorry for the impact to our customers and everyone who relies on them."
The company promised to examine and explain why it had failed to detect the software bug during its own testing process.
The incident has raised questions about the reliance of the internet on a few infrastructure companies.
Fastly is one of the world's most widely-used cloud-based content delivery network providers.