The Islamic State and other international terrorist groups are perfecting the use of drones to deliver explosives. Meanwhile, traffickers in the United States have already flown drugs over border fences and contraband over prison walls with drones. U.S. law could be falling behind the threat.

Federal officials testified Wednesday at a Senate Homeland Security and Governmental Affairs hearing that outdated statutes constrict the government’s ability to research and counter reckless or malicious drone use.

“This threat is real,” said David Glawe, undersecretary of intelligence and analysis at the Department of Homeland Security. “We are witnessing a constant evolution in the danger posed by drones as the technology advances and becomes more available and affordable worldwide.”

In response, Chairman Ron Johnson said he hopes to attach legislation to this year’s defense authorization bill that would give law enforcement agencies authority to curb the use of drones in U.S. skies.

Under the proposed legislation, DHS and the Department of Justice would gain enforcement authorities against drone use deemed to threaten public safety or national security. Law enforcement agencies could track or surveil drones in public areas, or even intercept, seize or destroy them if necessary.

Johnson said the bill would help address a “huge gap” between law enforcement capabilities and rapidly developing technology.

“I was shocked by the fact that we don’t have authority to counter this,” the Wisconsin Republican said. “We should thank our lucky stars that we haven’t seen a real tragic incident coming from this.”

Ranking member Claire McCaskill co-sponsored the legislation introduced by Johnson last month, along with two other Democratic senators — Sen. Heidi Heitkamp of North Dakota and freshman Sen. Doug Jones of Alabama. Four Republican senators signed on in support, including Florida’s Marco Rubio and Arizona’s Tom Cotton.

The American Civil Liberties Union opposes the bill, saying it lacks protections for “property, privacy and First Amendment rights.” 

“The bill amounts to an enormous unchecked grant of authority to the government to forcefully remove drones from the sky in nebulous security circumstances,” the ACLU wrote in a letter to Johnson and McCaskill on Wednesday morning.

McCaskill stressed the need to balance national security and public safety with concerns about civil liberties. She also noted the beneficial uses of drone technology, from recreation and photography to crop dusting and newscasting.

“We must encourage and foster that innovation in Congress,” the Missouri Democrat said. “But unfortunately, drones have the potential to cause great harm.”

Provisions of the bill were informed by the findings of an interagency committee, which identified barriers to the federal government’s ability to respond to drone threats, according to McCaskill.

Currently, a “whole host of statutes” constrain the authority of law enforcement toward drones, DHS deputy general counsel Hayley Chang said at the hearing.

Federal agencies can’t properly research or test systems for detecting or intercepting drones, according to Chang, because such systems remain illegal under federal law. Moreover, agencies fear that law enforcement officers could be exposed to criminal liability for trying to track or take down drones.

“Technology is evolving so rapidly, and the law is not keeping up right now,” Chang said.

Johnson illustrated the threat with footage of an ISIS drone flying over Iraq. In one of the terrorist group’s propaganda videos, an ISIS operative pilots an armed drone over an Iraqi military unit and detonates a grenade on a convoy sweeping for improvised explosive devices.

FBI Deputy Assistant Director Scott Brunner said ISIS and other overseas terrorist groups have “perfected” the use of cheap, commercially available drones for attacks and reconnaissance over the past two years — and groups may be interested in using these capabilities in the U.S.

“This is a significant threat to the homeland,” Glawe of DHS said. “We have to move with a sense of urgency.”