Interesting facts here are that with TCAS a TA occurs at approx 40 seconds from closest point of approach (CPA) depending on altitude related sensitivity and an RA is given 25secs before CPA. Considering the 50 second call not much room for manouvre is there.
One other point that appears to have not been considered in all the other posts is that of the collision height of 35250ft. Here the westbound aircraft had almost completed the necessary descent instruction but unfortunately the north bound aircraft had apparently initiated the TCAS descent requirement. However, everything should become clearer with the transcript of the CVR
As stated by teeteringhead and well covered by CRM courses lose a link and the chain is broken and in this case as others with dire effects.