Associated Press | Airline officials are trying to figure out how a sleeping passenger was left aboard a flight for four hours after it landed in Philadelphia.

According to police and the Transportation Security Administration, the passenger didn’t wake up when her United Express flight from Dulles airport outside Washington landed shortly after midnight Tuesday. About four hours later, a cleaning crew found her, according to KYW-TV.

United Airlines says they’re working with a regional partner carrier to determine why the plane wasn’t cleared upon landing.