A person gained access to the terminal because of a "procedure breakdown" between 3:30 a.m. and 4 a.m. today, airport spokesman Mike McCarron said.
Everyone in the United Airlines domestic terminal was cleared, and now have to go through security screenings again to reenter the terminal, McCarron said.
Passengers began reentering the terminal at around 5:30 a.m. today and the evacuation has caused long lines at security checkpoints.
McCarron said that the airport expects to have the lines thinned out over the next 45 minutes, and to be back to normal operations by around 7:15 a.m.
While he could not comment on the nature of the unauthorized access, McCarron said that airport officials are currently interviewing the person who entered the terminal.