I can move diagonally.

It might be possible for me to escape however if the policemen are stationed to the east and west of the mask it is impossible to take it and flee.

It takes one turn to move into one of the squares surrounding the mask.
It takes one turn to capture the mask.
It takes one turn to move away from the mask towards the exit.

The police can cover those squares in 2 turns. On one turn, they cover the east and west squares, and on the next turn they can see the thief, and capture him if he lands on the mask or an adjacent square. That would make the game unwinnable.