one of the men.............

。。。
One of the men that were caught has not signed a confession这句话中，that引导了一个定语从句修饰men。that指代men，the men that were caught 意思是“那些被抓了的人”。men是复数，又是过去时，所以后面的 be动词为were.

这里是虚拟语气，
例如
If I were ... I would

这是定语重句that代表one of the men所以单数用was

因为One of做主语时，无论后面的名词是单数还是复数，其做主语时，谓语动词要用单数，类似的还有every of,none of。