from previous....

.....he commented  to Peter  that he made a terrible mistake 

How did he know  it was him

Simple explained he to Peter,  it was his statement  in which he gad said  he was kept hostage  and when he escaped  John was shot down by the kidnappers,  but he was not told of it, so he was  from then on  kept under  surveillance,  though  it was a very simple  statement