11.   What enabled ants to flourish so quickly remains a mystery, however.

12.   What happened that July morning two years ago remains a mystery.

13.   Who stole the works, who returned them and where they have been over the past two decades remain mysteries.

14.   Who ordered the peaceful protest to turn violent and who signaled the first shot to be fired has remained a mystery.

15.   Why not remains a mystery.

16.   With the benefit of hindsight, the experts clearly guessed wrong, although it remains a mystery just how several postal workers got infected.

17.   But more than six weeks after she was found beheaded in the woods in Kansas City, her identity remains a mystery.

18.   Ground control did not receive any distress calls prior to the accident and the cause of the plane crash remains a mystery.

19.   However, much about the sound made by Parasaurolophus remains a mystery because the reconstruction is incomplete, Williamson said.

20.   On Tuesday, who fired the deadly shot and why remained a mystery.