Collocations for culprit - noun

big, chief, main, major, primary (esp. AmE), prime, real, true, worst
In the battle to stay slim, the biggest culprit is lack of exercise.

likely, obvious, possible

hunt, hunt for (both esp. BrE)
Police hunting the culprits have condemned the attack.

apprehend (esp. BrE), catch, discover, find, identify, track down