A House Homeland Security subcommittee today heard sharp warnings about the plan to allow drones to fly widely in U.S. airspace starting in 2015.

GPS expert Todd Humphreys of the University of Texas was the star witness.

“I am worried that it could be a weapon in the arsenal of organized crime, or state actors, or organized terrorists,“ Humphreys told a stunned committee.

In a Fox News exclusive last month, Humphreys demonstrated how, with a relatively inexpensive GPS “spoofer,” he could take control of a GPS-guided drone in flight, and make it do whatever he wanted. The potential is there, he told the panel, for terrorists to do the same thing.

Click here to read the exclusive report.

“The nightmare situation that I articulated here as a panelist,” Humphreys told Fox News, “was that five or ten years from now we haven't fixed the problem and now the drones are much larger, maybe delivering FedEx packages. I don't want it to get to that point before we say 'ok it's a problem.'”

Committee Chairman Rep. Mike McCaul of Texas was clearly alarmed by Humphreys’ testimony.

“This is astounding that you could hijack a UAV and bring it down,” he said.

McCaul was also visibly upset that the Department of Homeland Security refused his invitation to send a witness to today’s hearing, pointing out that the Government Accountability Office recommended in 2008 that DHS and the TSA take the lead in drone security.

“I am concerned DHS is reverting back to a pre-9/11 mindset, which the 9/11 Commission described as a lack of imagination in identifying threats and protecting the homeland,” he said in his opening statement.

The other committee members piled on. GAO official Gerald Dillingham was asked by Rep. Jeff Duncan, R-S.C., “Why has the Department of Homeland Security been so slow to develop policies and guidance related to the domestic use of drones?”

“That's a good question, sir,” Dellingham replied. “And we've not been able to get an answer from DHS or TSA why they have not followed our recommendations.”

Humphreys reminded the committee that military drones are not at risk, because they are guided by a GPS system that is heavily encrypted. And he outlined to the committee steps that could be taken to ‘harden’ the open-access civilian GPS system in a way to prevent “spoofing.” But he said it will take money, and acknowledged that budgets are tight.

“I believe it would fall to DHS to fund something like this,” he told the committee, adding, “I’m not terribly optimistic.”

While DHS would not appear at the hearing, a DHS official told Fox News that drones are the FAA’s jurisdiction, not theirs. The official also wondered why – if they are so concerned about the security implications now – members of Congress didn’t think about that before voting to open U.S. airspace to unmanned aircraft.

The anxiety at the hearing about drones wasn’t limited to safety and security. There are also huge privacy concerns – especially with new surveillance drones coming out that can stay aloft for 48 hours.

Amy Stepanovich of the Electronic Privacy Information Center testified that technology that allows drones to ‘see through walls’ with thermal imaging has far outstripped laws regarding privacy.

“Privacy laws are totally inadequate,” she told Fox News. “In fact, what we look at a quote-unquote reasonable expectation of privacy is totally going to be changed by the use of drones. So what we think of as our protections now are going to be totally different in 10 years.”

The committee also heard from law enforcement about how drones can be a ‘force multiplier,’ a comparatively inexpensive alternative to helicopters. But Prof. Humphreys told the panel that he was recently approached by Austin police about whether to use their drone on ‘game-day’ given concerns about public safety. Humphreys told them that if there is an active threat, by all means use it. If not, better to keep it on the ground.

The committee chairman – who voted to allow drones broad access to U.S. airspace in February -- acknowledged that he didn’t have all the information when he said “aye.”

He told Fox News, “I think maybe this is a little bit of an eye-opener for a lot of lawmakers, to be honest with you. The fact that there could be chaos in the sky, that eyes in the skies could invade on personal liberties and that there needs to be an agency that's in charge here.”