Bosses of Karachi police admitted in the Supreme Court on Friday that their men lacked the ‘will’ to combat the growing number of militants and gangsters in the city because of sluggish follow-up of the cases of their colleagues who had been killed by criminals.

The will was weak because cases of the killing of policemen were not followed up, Deputy Inspector General (DIG) Karachi West Zafar Abbas Bukhari said, adding that 162 policemen had lost their lives in different operations but only three killers could be arrested.
Link:http://dawn.com/2013/03/30/combating...mit-officials/

Alas no indication over what period this had occurred.