严格说来，They reported the loss to the police, in the meanwhile, they gave all the necessary particulars and evidence. 这个句子应作以下修改：

They reported the loss to the police. In the meanwhile, they gave all the necessary particulars and evidence.

They reported the loss to the police; in the meanwhile, they gave all the necessary particulars and evidence.

They reported the loss to the police, and in the meanwhile they gave all the necessary particulars and evidence.

但在现代英语中，有时也有人会省去其中的连词and，构成所谓的逗号句。

有关英语逗号句的问题，我在“英语语法网”读过陈老师一篇专门的文章，写得不错，你可以参考一下：http://www.yygrammar.com/Article/201311/3526.html