Who is responsible for the DOS attack against Twitter

I have been trying to post for the last one or two hours when I found out the reason for this

Thu

Ongoing denial-of-service attack 1 minute ago

We are defending against a denial-of-service attack, and will update status again shortly.

Site is down 1 hour ago

We are determining the cause and will provide an update shortly.

Update: we are defending against a denial-of-service attack.

keep up to date with the situation with this link - http://status.twitter.com/

Could it be the Iranians? or some other person or company that is aggrieved by the truth being posted on the site and they wish to suppress the truth.