In 1963, there were roughly 411 unsolved murders in the United States each year, and about 4,155 murders were solved each year.
In 2007, there were roughly 6,073 unsolved murders in the United States each year, while approximately 8,738 murders were solved.
Unsolved murders are up by a factor of more than 14. Solved murders have more than doubled. The population of the United States has increased by about 60% (from 189 million to 301 million) from 1963 to 2007.
Thus, the per capita rate of solved murders has increaesd by 32% over 44 years, while hte per capita rate of unsolved murders has increased by 828% in the same time period. The rise in unsolved murders has been attributed by some commentators to drug trade and gang related murders, like drive by shootings, that are hard to solve.
Notably, this time period coincides with stepped up efforts to regulate illegal drug use.