Pakistan officials are refusing to release the British-born militant found guilty of a US journalist's murder - despite judges throwing out his conviction.
Ahmed Sheikh had his sentence reduced to seven years for kidnapping by judges in Sindh High Court on Thursday.
He had been on death row for the killing of Daniel Pearl since 2002.
Sheikh's lawyers believed he would be free in "days", but Sindh officials ordered three more months' detention.
A Sindh government official said they had concerns that Ahmed Omar Saeed Sheikh and three other acquitted men, who were serving life sentences for their roles, may act "against the interest of the country".
The law being used to detain the men in prison has previously been used to keep other high-profile militants detained, Reuters news agency reported.
The decision came after the US's top diplomat for South Asia decried the acquittals as "an affront to victims of terrorism everywhere'.
What happened to Daniel Pearl?
Pearl, who worked for the Wall Street Journal, went missing in January 2002.
He had been researching links between Islamist militant activity in Karachi and Richard Reid, who tried to blow up a passenger plane using bombs hidden in his shoes.
According to prosecutors, Sheikh lured him to a meeting with an Islamic cleric. The two had built a relationship discussing concerns about their wives, who were both pregnant at the time.
Almost a month later, a video showing the 38-year-old's beheading was sent to the US consulate in Karachi.
Pearl's son, Adam, was born in May 2002.
Sheikh was convicted of Pearl's murder in July 2002 by an anti-terrorism court, and has been on death row since.
Who is Ahmed Omar Saeed Sheikh?
Sheikh was born in London in 1973, where he attended public school before going on to study at the London School of Economics. He did not graduate, failing to return after driving aid to Bosnia after his first year.
He was arrested for being involved in the kidnapping of four tourists - three British and one American - in Delhi in 1994.
He was released from prison as part of demands by militants who hijacked a plane in 1999.
According to news agency Reuters, police in India later accused him of transferring money to one of the militants who flew a plane into the World Trade Center in New York on 9/11.