Twitter has said an attack on Thursday which took the site offline for many users was the result of a DNS redirect.
A group calling itself the Iranian Cyber Army redirected users to a defacement page showing a flag, according to a screen grab on Flickr.
Twitter said in a blog post on Friday that the majority of users had not been able to access Twitter for over an hour.
"Last night, DNS settings for the Twitter web site were hijacked," said the Friday blog post. "From 9:46pm to 11pm PST, approximately 80 percent of Traffic to Twitter.com was redirected to other websites."
However, Twitter said it did not believe user accounts had been compromised.
The attackers used internal credentials. according to Wired.
In January 2009 Twitter was subjected to a brute-force dictionary attack which led to a break-in of celebrity Twitter accounts. The attacker managed to gain internal credentials - an administrator's password.
The Domain Name System (DNS) is often likened to a phone book for the internet. DNS is the protocol used to translate human-readable URLs into numeric IP addresses.