Captain John Kirby, spokesman for the United States Department of Defence has tweeted that the reasoning behind the shootings of two ISAF military advisers is not yet known.

In a tweet on his official account, Kirby acknowledges the speculation around today's shooting.

Lots of speculation on today's attack in Kabul. We do NOT know who killed 2 ISAF members or why. Thoughts and prayers to loved ones.

— CAPT John Kirby (@DoDSpokesman) February 25, 2012