Read the sentence to find out whether there is any grammatical error in it. The error, if any, will be in one part of the sentence. Mark the part with the error as the answer. (Ignore the errors of punctuations, if any).
Police officers have refused on identify the bystander, / who is the only eyewitness to the crime, / but have said that the investigating team would explore / if he could be a witness in the case.