Retired Coast Guard Admiral James Loy has called the US marine transportation system a terrorist’s “natural gateway into America.’’ And those gates remain open so long as only a small fraction of cargo containers are scanned before arriving in US ports.

Security experts worry most about the possibility that weapons of mass destruction, including nuclear devices, could be secreted aboard one of the thousands of container ships entering US ports each year. Such concerns led to a 2007 law co-authored by Representative Edward Markey requiring all US-bound containers to be scanned before departing from overseas. But the July deadline won’t be met. And the decision to extend the deadline by two years won’t advance the cause too much if the pace of the last few years continues.

Currently, only about 5 percent of the containers entering the country are scanned. That’s not quite as scary as it sounds. US Customs and Border Protection uses multiple intelligence methods to select containers for inspection, concentrating both on ports that ship large number of containers to the United States and on areas where terrorists are known to operate. Department of Homeland Security officials maintain that “100 percent of high-risk containers’’ are inspected through this “risk-based’’ approach.

The system has held up well, so far. But the current rate of inspection is too low, especially given the enormously high stakes: possible detonation of a radiological device. That argues strongly for the timely creation of a global container inspection system — with special emphasis on radiation detection — for every container entering a US port. It is possible to scan 100 percent of containers arriving by truck for loading onto ships without disrupting productivity, as shown in a Hong Kong pilot program.

Homeland Security has missed deadlines for implementing 100 percent cargo screening on US-bound international flights. But the agency always appeared to be pursuing that goal aggressively. What’s worrisome now is that federal security officials don’t appear vaguely ready — or even willing — to apply the same standard to cargo that arrives by sea.