The country's interior minister says the attack was carried out by an "Islamist terrorist" who was shot dead by police after a rampage through central Vienna.
Thousands of demonstrators gathered in solidarity in French cities on Sunday, as the number of people detained in connection with the history teacher's beheading rose to 11.
Authorities say an 18-year-old Chechen refugee attacked the teacher after a lesson in which caricatures of the Prophet Muhammad were shown. Among those arrested were members of the suspect's family.
A Kenyan court found two suspects guilty and acquitted another for their roles in the 2013 attack. The men were charged with conspiring to commit terrorism and aiding a terrorist group.