Around March 10 when he questioned Attorney General Loretta Lynch at a DOJ budget review hearing [1]
The video is 6 minutes and worth watching. It sounds like he is still forming his whole opinion. He certainly has changed his view from his initial comments. He's now calling for the debate to be held in Congress via passage of new laws.
".@tim_cook Our nation is at war & this Iphone was used to kill Americans. Protect our homeland, not terrorists. Please cooperate with @FBI."
https://twitter.com/GrahamBlog/status/700348813807063040 http://www.lgraham.senate.gov/public/index.cfm/