The Department of Homeland Security uncovered more than 60 cross-border tunnels along the southwest border that were used to smuggle people and illicit drugs into the United States over a five-year period, according to a new government report.

The agency also detected more than 530 "ultralight aircraft" intrusions into the United States and roughly 300 drug smuggling incidents involving small fishing boats and recreational vessels along American borders from 2011 to 2016, the Government Accountability Office reported.

All 67 tunnels discovered by border patrol agents were located along the U.S.-Mexican border. Likewise, all but one of the light aircraft incursions was detected on the southwest border in Arizona, California, New Mexico, and Texas.

A majority—nearly 76 percent—of the 309 maritime drug smuggling incidents involving fishing boats occurred on the West Coast, while the remaining 24 percent took place on the southeast coast, northeast coast, and southwest border.

The GAO report noted that while tunnel discoveries and detections of ultralight aircraft intrusions and smuggling incidents on small boats declined over the five-year span, the routes remain a threat to national security, particularly if used by militant groups to smuggle terrorists and weapons into America.

"While these methods account for a small proportion of known smuggling, they can be used to transport significant quantities of drugs or for terrorist activity," the report noted.

Federal officials said transnational criminal groups constantly alter their smuggling methods as the United States cracks down on border activity to evade detection, posing a constant challenge to DHS officials.

Though the agency has made significant investments in personnel, technology, and infrastructure to prevent smugglers from entering into the country, DHS officials have no way of determining whether their efforts are working, according to GAO.

"Agency resources are being invested to address cross-border tunnels, ultralight aircraft and selected maritime smuggling methods and without some type of performance measurement, DHS does not have reasonable assurance that efforts to address these selected smuggling methods are effective," the report said.

GAO urged DHS to implement standard operating procedures across the agencies involved in detecting and preventing smuggling attempts, including Immigration and Customs Enforcement and Customs and Border Protection, so that the department can better facilitate information-sharing and adequately measure their efforts.