Meaning?

Hi, would you please tell me what is the meaning of you are playing his game in the following text:

" And you are certain it was the killer?"

"No. we"re certain it was the man who dropped off the cell phones ( in the post office box). Probably the killer but we are not sure."

" Right. why would he drive up there to ditch the phones?"

The sheriff said, " Now you are playing his game. I think he was just having some fun with us.... "

Thanks