The SSS released Salihu Tanko Monday evening.
The State Security Service (SSS) has freed Salihu Tanko, a former media aide to the Kano State Governor, Abdullahi Ganduje.
Mr Tanko was arrested on Friday after calling on President Muhammadu Buhari to check Nigeria's worsening insecurity or resign.
He was sacked by Governor Ganduje shortly after his arrest.
Mr Tanko's father, Tanko Yakassai, confirmed to PREMIUM TIMES that he was released late Monday evening.
The elder politician said he was yet to be briefed on the alleged crime for which he was arrested and detained by the SSS.
PREMIUM TIMES reported Mr Tanko, on Twitter, saying the All Progressive Congress (APC) has failed Nigerians over the worsening insecurity.
Following the abductions of 27 schoolboys in Kagara, Niger State and 317 schoolgirls in Zamfara, Mr Tanko took to social media to express his anger.
He said the APC government "at all levels" has failed to deliver on its primary responsibility of protecting the lives and properties of Nigerians.
Mr Tanko, in a series of tweets, called on President Buhari to deal with the escalating insecurity or resign.
"Clearly, we as APC government, at all levels, have failed Nigerians in the number 1 duty we were elected to do, which is to secure lives & properties.
"Not a single day goes by without some sort of insecurity in this land. This is a shame! Deal with terrorists decisively or resign," he wrote on his verified Twitter handle, @dawisu
Governor Ganduje relieved Mr Tanko of his appointment after his arrest.
A statement by the state government announcing the sack said "he failed to differentiate between personal opinion and official stand on matters of public concern and therefore could not be allowed to continue to serve in a government he does not believe in".
Confirming Mr Tanko's arrest, the SSS spokesperson, Peter Afunnaya, in a short statement late Saturday in Abuja said he was in their custody, facing investigation.
"This is to confirm that Salihu Tanko-Yakasai is with the Department of State Services. He is being investigated over issues beyond the expression of opinions in the social media as wrongly alleged by sections of the public," the statement said.
The secret police has not released Mr Tanko.