The decision not to file fresh charges against separatist leader Masarat Alam was taken weeks before the Mufti Mohammad Sayeed government came to power, during the 49-day Governor's Rule in Jammu and Kashmir.
In a letter accessed by NDTV, the state's Home Secretary Suresh Kumar wrote to the Jammu District Magistrate on February 4 saying the detention order against Mr Alam under the Public Safety Act (PSA), issued in September 2014, had become void.
This was because the order had not been confirmed by the state Home Department within the mandatory 12 days of issuance of the order under the provisions of the Act.
"The period of twelve days in the instant case has already elapsed, as such, the detention order has become non-est in the eyes of law," the letter said.
Moreover, the advisory board had not approved this case within the stipulated period of one month.
The release of Mr Alam by the Peoples Democratic Party-led government on Saturday has unleashed a political storm. Accused of fanning the 2010 street protests in which over 100 people died, Mr Alam, 44, had been repeatedly detained under the Public Security Act for four years.
The state BJP, which has already complained to Chief Minister Mufti Mohammad Sayeed, also wants all cases against Mr Alam to be investigated afresh.
Read the Home Secretary's letter to the District Magistrate:
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