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Hussein Mahdi had previously targeted other Iraqi government websites to air grievances against the authorities, who have now responded similarly with posts on his social media accounts.

"Not caring about the lives of the people is one of the most important reasons for the destruction of this country," Mahdi on the agency's hacked webpage.

"The National Security Service is neither national nor secure, unfortunately it is corrupt and cronyistic," he said.

He specifically criticised the agency's alleged reliance on Facebook for intelligence information as well as the monitoring of Friday prayers and of Shia mosques.

The hack came soon after a suicide bombing ripped through a popular Baghdad ice cream shop earlier this week, although it was unclear if the failure to prevent the attack inspired Mahdi's actions.

The National Security Service has since announced his arrest, saying in a statement that he is accused of "hacking its website and other official sites".

	A screengrab of Mahdi's hacked message

After his detention, a message telling people that "the National Security Service is your service" and that "attempting to hack its website is in the interest of the Dawaesh and the enemies of Iraq" appeared on Mahdi's Facebook and Twitter accounts.

"Dawaesh" is a pejorative term for members of the Islamic State group, the Sunni extremist organisation that Iraq is battling in the northern city of Mosul and other areas.

Mahdi, however, is a member of Iraq's Shia majority, which IS has frequently targeted in bombings and other attacks.

An official from the agency said that it posted the messages on Mahdi's social media accounts after confiscating his phone.

But his detention did not have the preventative effect that the agency might have hoped: the websites of the ministries of housing and construction, municipalities, youth and sports and Basra University were subsequently hacked.

"The goal of the hacking is not sabotage" but rather to call for Mahdi's release, posts on the hacked websites said.