DOD--at least according to the AP story--won't investigate until there's evidence US personnel were present. Yet survivors of the massacre did describe Americans being present at least when the Afghans were loaded onto trucks.
Now, the investigative report Salon
http://www.salon.com/news/primary_sources/2009/07/22/mass_graves/index.html includes describes one American present before the Afghans were put into shipping containers, and Americans arriving at the prison a month after the Afghans were saved from the shipping containers. So it may be that any Americans were simply working with Dostum more generally (and photographing captivity operations). But it seems like the reports ought to be enough to require DOD to investigate.
Read the whole story--there's a very disturbing report of the Taliban who currently hold a US solder captive invoking Dostum. Which might explain why this story originally got dumped into a Saturday news black hole.