National privacy laws aren't ready to handle the 10,000 commercial drones predicted to be flying around in US airspace by 2020, nor the surveillance drones that some local police and US law enforcement agencies are using already and that many more are seeking. That's the conclusion voiced by US Senators and various expert witnesses during a hearing on domestic drone usage this morning.

"We need to do more to prevent drones from being used in an invasive manner that violates Americans' privacy rights," said Sen. Al Franken (D-MN), chairman of the Senate's subcommittee on privacy, later adding that he hoped his fellow lawmakers would "continue to work with me on appropriate legislation" to govern drone usage in the US. Already, several members of the House of Representatives have introduced bills targeted at imposing restrictions on drone usage.

"We need to do more to prevent drones from being used in an invasive manner."

Although no major privacy violations were cited in the hearing, the Senators in the Judiciary Committee hearing raised a number of outlandish hypothetical nightmare scenarios, such as drones being used by terrorists, drug traffickers and even pedophiles. They also posed a number of outstanding questions that still aren't clear under US law, including: "What kind of altitude can [drones legally] fly?" as Senator Diane Feinstein (D-CA) asked.

The Federal Aviation Administration currently issues permits allowing police departments to fly unmanned aircraft below 400 feet. A 1989 Supreme Court ruling upheld that a manned police helicopter flying at just that height didn't violate privacy laws. However, "it's an open question whether lower surveillance vehicles would be included in the definition of what's a reasonable expectation of privacy," said Amie Stepanovich, an attorney with the Electronic Privacy Information Center. Settling these issues is a time-sensitive matter: drones for commercial usage are currently banned from flying in the US, but Congress passed a bill in February that will effectively legalize their use by 2015.

"It's an open question whether drones flying below 400 feet violate privacy."

Benjamin Miller, a member of the sheriff's office of Mesa County, Colorado, population 175,000, testified in the hearing that his department has been using two consumer drones outfitted with cameras — the Draganflyer X6 and Falcon UAV — on various operations over the past four years. The agency has used drones for everything from pinpointing areas of a church that were still hot after a fire, to helping locate a missing elderly woman's body. Not surprisingly, Miller said he didn't think Congress should legislate heavily to regulate drone usage in the US. "I think the limitations we'd support are the ones we have currently identified through case law," Miller told the Senators.

"There is a huge upside to this technology."

Michael Toscano, the President and CEO of a drone lobbying group, the Association for Unmanned Vehicle Systems International (AUVSI), echoed that sentiment and went further, pointing to a study his group recently released that found the unmanned systems industry could create over 70,000 jobs in three years and add $10 billion to the economy, with most of the new commercial drones (80 percent) being used for agricultural surveys. "There is a huge upside to this technology," Toscano said during the hearing, but he acknowledged there were concerns: "You cannot stop people from misusing any type of technology."

While the present-day costs and benefits of drones drove the debate, Senators and witnesses alike frequently looked to the future, when the dropping costs and rising capabilities of unmanned vehicles could enable persistent surveillance or weaponization. Miller firmly denied that lethal weapons could be feasibly used on police drones, and he said that long-term surveillance like that done by military drones was "not affordable." But as Senator Leahy and others noted, there's no guarantee it will remain so. GPS tracking, which was recently the subject of a major privacy case, was mentioned as a point of comparison: when a technology is cheap and simple to use, it's tempting to abuse it.

One thing that the witnesses seemed to reach a rough agreement on was that any effort by Congress to update the laws shouldn't target drones per se, but the application of new surveillance technologies more broadly. "I really believe we should be updating Fourth Amendment law in general to deal with surveillance technologies," said Ryan Calo, a professor at the University of Washington law school who specializes in technology law.

Adi Robertson and Ben Popper contributed to this report.