As with most tree troubles, there is no way to diagnose without first identifying the species, taking a first hand look at the site and obtaining a full history, however I am highly suspicious that your tree may be infested with the Douglas-fir Tussock Moth.  The Douglas-fir Tussock Moth affects Douglas-fir, white fir, and blue spruce in Northern New Mexico, and can easily remove all foliage in a few years, usually starting at the top.  The overwintered eggs hatch from mid May to early June and the baby caterpillars feed on the new growth.  They are compelled to climb as high as they can to release a silk parachute to carry them away in a breeze in hopes of finding a new host tree, but most perish on the journey.  This compulsion of the young to climb upward may be what causes just the top of the tree to die, but it is actually the mature Tussock Moth that does the most damage which can kill the rest of the tree.