Question

Use commas to set off direct quotations from the rest of the sentence.

"I know you did it " the detective accused the murderer "and I can prove it!"
"Please remember to watch the gap" the conductor warned.

Answer 1

In English, commas should be used within a quote, in order to separate it from the indirect speech, and before the quote, for the same reason.
Thus, this is the correct answer:
"I know you did it," the detective accused the murderer, "and I can prove it!"
"Please remember to watch the gap," the conductor warned.