According to the FBI Denver Twitter account, there is the risk hackers have targeted the power points to put malware or monitoring software on you phone.
The warning also extends to power points at hotels and shopping centres.
"Avoid using free charging stations in airports, hotels or shopping centers," it said.
"Bad actors have figured out ways to use public USB ports to introduce malware and monitoring software onto devices."
The FBI recommends bringing "your own charger and USB cord and use an electrical outlet instead".
The FBI Denver Twitter account regularly posts advisory tweets with information to help keep people safe.
