One of the most mysterious unsolved bank robberies took place at the First National Bank of Chicago, USA. One Friday in 1977, an employee at the bank put $$4 million in cash in a bank vault. When another employee came to make the weekly check on the vault, she discovered that only $$3 million was there. A million dollars had simply disappeared. The robbery appeared in all the national newspapers. Because nobody had broken into the vault, the police suspected it was an 'inside job, and were at first hopeful that they would recover the money quickly. 'It's foolish to rob your own bank,' said one police officer. 'You are almost certain to get caught, unless you are very lucky.' Because most robbers are in a hurry, they are usually careless and leave clues, but the police could find nothing to identify the thief or thieves. The FBI were suspicious of one employee, but they didn't have enough reliable evidence to arrest him. Four years later, police discovered $$2,300 of the money in a drugs raid, but have never found the thief or the rest of the money.