A spokesman for NATO's International Security Assistance Force (ISAF) said the crash on 16 August south of Herat was likely an accident.
But Spanish Defense Minister Jose Bono said before flying to Afghanistan to oversee the investigation that hostile fire could not yet be ruled out as the cause of the crash.
"We believe it is best not to rule out that it could have been an accident caused by technical reasons, nor can we rule out that it could have been an external attack," Bono said.
Investigators are also looking into the cause of an emergency "hard landing" of a second Spanish helicopter nearby shortly after the crash. Five Spanish troops were slightly injured in that incident.
In a separate development, Afghan officials are trying to win the release of Mohammad Reza, a Lebanese engineer kidnapped in Zabul Province on 14 August.
On 16 August, a man claiming to be a Taliban spokesman, Abdul Latif Hakimi (aka Mufti Latifullah Hakimi),
said Reza will be killed unless his Turkish employer withdraws from Afghanistan by noon today local time.
(Reuters, AFP, AP, RFE/RL)