The active shooting that killed five at Fort Lauderdale’s airport serves as a stark reminder of the challenges of providing security outside checkpoints at sprawling airports.

Transportation Security Administration officers scrutinize passengers before they board planes, but those officers are unarmed and located inside the airport beyond airline check-in desks. Armed local police routinely patrol public areas of airports, but sparsely compared to TSA staffing.

Jeh Johnson, secretary of Homeland Security, has moved to limit airport terminal entrances and have more uniformed patrols of public areas in airports, such as arrivals and baggage claim.

But the lack of security at terminal doors is a vulnerability that officials have long acknowledged, but that is difficult to remedy because of the prohibitive cost of staffing every entrance, USA Today reports.