Prompt, credible and impartial investigation

Hilary Clinton said that the US had agreed to a prompt, credible and impartial investigation on the Israeli attack on Aid Ship heading to Gaza. This is her response to the UN's call for an investigation.

I am wondering why the UN did not call for a prompt, credible and impartial investigation. And I am wondering why the US, UK and the South Koreans went ahead on a partial investigation without