I had to subtract half a star for this:
In Author Comments you said "police is" when you should have said "police are."
"is" is the third person singular present of "be"
"are" is the plural present of "be"
Police is a plural noun referring to a group of policemen and women.
You might modify the clause to "The policeman is convinced Charles Williams has murdered Jimmy Johnston,"
But I am going to assume you intended on referring to the plural "police" in which case the clause should have read:
"The police are convinced Charles Williams has murdered Jimmy Johnston,"
I used these resources:
also.. game good