It's a bit suspicious that no one knows where he was at the time of the murder.

suspicious 是形容词，由名词 suspicion（怀疑）加上形容词后缀 -ious 构成，表示"可疑的、引起怀疑的"。这个词描述一种让人产生不信任感或觉得可能有问题的状态。