(NBC News) Investigators are zeroing in on an argument that reportedly took place shortly before Wednesday's shooting spree at Fort Hood. It may the so-called "trigger factor" they're looking for.
Investigators say the gunman, Army Specialist Ivan Lopez, was being treated for mental illness and that may have been an underlying factor in the shooting.
Lopez was reportedly upset about not getting more time to attend his mother's funeral.
A memorial service to honor the fallen and the wounded is planned for early next week.
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