Security checkpoints were used to cut the Crimean Peninsula off from the rest of Ukraine and to restrict movement within the territory.
In April, began in eastern Ukraine between Russian-backed separatist forces and Ukrainian government.
Polk was a professor of history at the University of Chicago.
Further information: , , and A Russian missile attack on in the city of took place on 8 April, reportedly killing at least 52 people and injuring 87 to 300 people.
All vehicles and tanks were unmarked.
Tensions over the issue had been rising for months.