CBI, on Saturday, Dec 4, registered the case under various sections of Information Technology Act.
CBI officials said that they are working to restore the website, which has gone offline after the attack. The hackers, who identified themselves as "Pakistani Cyber Army" have infiltrated the root server.
"It has come to the notice of the CBI that its official website was unauthorisedly accessed and defaced in the intervening night of December 3 and 4," CBI spokesperson said in a statement.
"A case has been registered in this connection in the cyber crime cell of the CBI and efforts are underway to restore the website with the help of National Informatics Centre and the CBI cyber security experts," the statement added.
Pakistani hackers attacked the CBI website on Friday, Dec 3 night and left note saying that it was a response for the attack of Pakistan websites by Indian hackers.