US Customs and Border Protection (CBP) officers do not have the authority to rifle through data stored solely on remote servers. The agents are looking for contraband (e.g. child pornography), information indicating inadmissibility, or information that could present a threat to national security. Agents can search a traveler’s phone without consent, but only for data saved directly to the device. US citizens can t be prevented from re-entering the country even though they decline to unlock the device, CBP says. Senator Ron Wyden has introduced the Protecting Data at the Border Act.
Source: https://www.helpnetsecurity.com/2017/07/17/cloud-data-us-border/