Bristol, VA -

Alpha Natural Resources says they've made safety improvements to their coal mines after purchasing Massey Energy.

The Bristol, Virginia based mining company sent a letter to federal prosecutors saying the changes were to avoid coal dust violations.

Investigators say coal dust caused the explosion at Upper Big Branch that killed 29 miners when it was owned by Massey.

The changes are part of a $210 million settlement. Alpha is expected to release more information on efforts to reform former Massey operations next month.