Abdur Rehman Sindhi was detained during a raid by intelligence agencies in the southern port city of Karachi last week, said police officer Muqaddas Haider.
He said a joint team of police and intelligence agents was questioning the suspect on what role he might have played in militant attacks in Pakistan in recent years.
Sindhi appeared before a court which allowed the police to interrogate him for two weeks, the police official said.
He said he didn't have any evidence so far that the suspect was linked to the US reporter Daniel Pearl's 2002 killing.