If they die within a few seconds of you. you get the kill, if not. then its a suicide for them.
At least judging by the game's kill mechanics thats how it "should" work.
for example. if you track someone while they are in deep water, you get the kill for drowning them. or if you shoot someone near a cliff and they drive off and die, you get the kill. Instead of it being a suicide for them.
A similar situation has happened to me on a rare occasion where I crashed into someone's bum and lit them on fire, but I killed myself (which the enemy got the kill credit for). they died by fire and I got the kill from the grave. (proves killing from grave is possible)
I believe the ruling is somewhere within 10 seconds of damage(probably <5). Tracking them INTO deep water (as tanks sometimes spin out when you track them) will give them a suicide.