1.find out where the responsibility lies;
2.follow up a clue; trace a clue;
3.trace a rumour to its source
6.anyone who did was tracked down, stigmatized and attacked politically.
7.We are running down a few tips.
8.I have decided not to pursue (ie investigate) the matter any further
9.dig out and trace all such samizdat literature
10.The rumor has been traced back to a bad man.