Brasilia was once spared the high crime rates in other big cities in Brazil like Rio, but recently crime has been on the rise. Tourists should practice extreme caution while traveling in Brasilia. Pickpockets tend to target places where tourists congregate, so be cautious when going to historic sites, museums, restaurants, or when taking public transportation. Keep your most important documents such as your passport close to your body like in a front pocket rather than in a back pocket or in a backpack or other bag. Don't keep all your money in one place, like in your wallet. Keep money in your front pocket along with credit cards, where it is more difficult for pickpockets to get to. Always keep an eye on your belongings. Don't leave them unattended for even short periods of time, as this is when thieves strike. When using ATMs, be wary of strangers who get too close to you while you are entering your PIN. Avoid walking alone in isolated areas, especially after dark.
One type of crime that has been used with more frequency is "quicknappings," in which the victim is kidnapped outside of a bank or ATM and then returned in a short amount of time in exchange for a payment from the victim's family or from the victim himself.
For more information, see the US State Department's Brazil travel page: