A computer system glitch has caused big delays at customs and border control at US airports.
In cities including Miami, Los Angeles, Boston, New York and Atlanta, there were long lines of travellers affected.
The glitch lasted several hours but is not thought to have been caused by anything suspicious.
One traveller in Boston said: “It looked like the machines were broken, the green cards weren’t working. It was a mess in there.
“I don’t know, it took forever, an hour and a half maybe to get through the whole thing. I’m ready to go home.”
The delays came at the worst time of the year, as people return from holiday breaks.
All airports were declared back on line after about four hours.
Officials say during the disruption people were still screened according to strict security standards.