Following the tragedies of September 11, 2001, contemporary artists such as Ahmed Basiony, Thomas Demand, Harun Farocki, Jenny Holzer, Trevor Paglen and Taryn Simon urgently pursued the complicated intersection of freedom, security, secrecy, power and violence. 'Covert operations: Investigating the known unknowns' features 13 international artists who have collected and revealed unreported information on subjects ranging from classified military sites...