According to tech blog Mashable, this was the most prolonged outage in eight months, as Twitter was down for more than an hour.
CNN Money said a hacker from the group UG Nazi claimed responsibility for the outage.
The hacker, who calls herself Cosmo, said the site was taken down by a simple DDoS or distributed denial of service attack. She said, "It wasn't really difficult at all, I myself honestly thought Twitter would be more protected from a DDoS Attack, but I guess not," according to The Telegraph.
Cosmo, also known as Hannah Sweet, said that the group attacked Twitter because it supported Cyber Intelligence Sharing and Protection Act (CISPA), CBS News reported.
The bill allows businesses and governments to share information on cyber threats but has been criticized for being too vague.
The Associated Press said there was no indication that the outage was due to malicious attacks.
During the outage, Twitter said, "Users may be experiencing issues accessing Twitter. Our engineers are currently working to resolve the issue."
Twitter tweeted this update once the site was back up:
Today's outage is due to a cascaded bug in one of our infrastructure components. We'll provide updated information soon.