Linguistic question re IV

[David Morris]

The police know he was there, but they need a witness, someone willing
to be on record.

2011/8/9 János Széky <miksaapja at gmail.com>:
> There is Jade apologizing on page 84: "...the cops told us they'd drop
> charges if we just put you at the scene, which they already knew you
> were so where was the harm..."
> The syntax seems to be elliptical here so please tell me what 'which
> they already knew you were' means exactly.
>
> Thanks,
> János
>