You can start this mission at any time once you start to unlock the ctOS towers throughout the city. However, you can only find 5 of the 7 bodies right without having to progress through the campaign. The final 2 bodies are hidden until Act 3 Mission 3-4.
. After those are revealed, you will get a phone call that you trace and find a 'crime alert'. Proceed to the crime and stop the killer. The achievement should pop shortly there after.