A week ago the US Transportation Safety Administration announced that for certain international direct flights bound for the US, they will be checking all electronic devices.
According to the announcement, security officers may ask you to power up your device. And if the battery is dead, they say, "Powerless devices will not be permitted onboard the aircraft." And, "The traveler may also undergo additional screening." The concern, according to NBC News, is that western fighters in Syria may try to enter the US concealing bombs in electronic devices. Although the TSA isn't giving specifics, the screening is reportedly taking place in airports in cities such as London, Frankfurt, and Paris. And while the TSA doesn't have the authority to govern security measures in those airports, apparently the threat of flights being canceled by Homeland Security encourages them to follow the guidance being given.
According to the NBC report, the Apple iPhone and Samsung Galaxy phones are receiving special scrutiny on flights from Europe, the Middle East, and Africa. Also being checked are laptops, tablets, and other electronic devices.
So be sure your devices are charged and ready to perform for security personnel if you're taking a flight from those locations to the US.