The EFF has some advice for interacting with border agents. It is always better to decline answering than it is to give a false answer. Don’t obstruct an Agent s investigation or attempt to delete data once it becomes clear that an agent intends on examining a device. Only a judge can make you reveal a passphrase or otherwise decrypt a machine. Refusing to cooperate may lead to the indefinite detention of your device and you may not be allowed in the country. The EFF says that refusing to cooperate could lead to an indefinite detention.
Source: https://threatpost.com/know-who-youre-dealing-with/91902/