警方决定对抢劫案展开调查。

The police decided to carry out an ___investigation into the robbery.

对一名十三岁男孩失踪案的调查已经进行了好几天。

An ___investigation has been under way for several days into the disappearance of a thirteen-year-old boy.

为确定事故原因，开展了一次彻底的调查。

A thorough ___investigation was launched to determine the cause of the accident.