The expanding options for communicating over the Internet and the increasing adoption of encryption technologies could leave law enforcement agents “in the dark” and unable to collect evidence against criminals, the Director of the FBI said in a speech on Thursday.

In a post-Snowden plea for a policy more permissive of spying, FBI Director James B. Comey raised the specters of child predators, violent criminals, and crafty terrorists to argue that companies should build surveillance capabilities into the design of their products and allow lawful interception of communications. In his speech given at the Brookings Institute in Washington DC, Comey listed four cases where having access to a mobile phone or laptop proved crucial to an investigation and another case where such access was critical to exonerating wrongly accused teens.

All of that will go away, or at least become much harder, if the current trend continues, he argued.

“Those charged with protecting our people aren’t always able to access the evidence we need to prosecute crime and prevent terrorism even with lawful authority,” Comey said in the published speech. “We have the legal authority to intercept and access communications and information pursuant to court order, but we often lack the technical ability to do so.”

Following the leak of classified documents by former National Security Agency contractor Edward Snowden, the public has become increasingly wary of overreaching surveillance by government agencies. Anti-spy technology has become much more popular — gaining popular support on Kickstarter, for example — and recently Apple and Google both announced that encryption would be the default for their mobile operating systems.

Both law enforcement and intelligence agencies have complained that such technology will curtail their ability to do their job. In essence, US law enforcement and national security agencies want citizens to accept spying as a possibility and to rely on policy, rather than on technology, to make sure that it’s lawful.

Civil-rights supporters argue that law enforcement will be able to do their job, and any assertions that they cannot are disingenuous.

“Whether the FBI calls it a front door or a backdoor, any effort by the FBI to weaken encryption leaves our highly personal information and our business information vulnerable to hacking by foreign governments and criminals,” Laura W. Murphy, director of the Washington Legislative Office of the American Civil Liberties Union, said in a statement.

Comey targeted recent moves by Apple and Google to encrypt user data on their devices, saying that while much of the data is backed up to the cloud — and thus accessible by law enforcement with a search warrant — other data is not, creating a “black hole for law enforcement.”

“Both companies are run by good people, responding to what they perceive is a market demand,” he said. “But the place they are leading us is one we shouldn’t go to without careful thought and debate as a country.”

Reacting to consumer sentiment, Apple announced in September that its latest iOS 8 operating system would enable encryption by default, making it impossible for law enforcement — or even the iPhone maker — to decrypt user data. Google followed suit a day later, pledging to turn on encryption by default in the next version of its Android operating system.

To a public increasingly sensitive to privacy issues, these are positive steps.

“We applaud tech leaders like Apple and Google that are unwilling to weaken security for everyone to allow the government yet another tool in its already vast surveillance arsenal,” ACLU’s Murphy argued. “We hope that others in the tech industry follow their lead and realize that customers put a high value on privacy, security and free speech.”

Google stressed that encryption is similar to older practice of putting documents in a vault or safe.

“People previously used safes and combination locks to keep their information secure—now they use encryption,” the company said in a statement sent to Ars Technica. “It’s why we have worked hard to provide this added security for our users.”

Apple did not immediately respond to a request for comment.

Comey stressed that neither he, nor the FBI, has the answer to these thorny issues. Yet the public needs to resolve the questions, he said. In addition, the pendulum of public opinion — which had swung far to the side of security following 9/11 — has now swung too far in the opposite direction, he said.

“My goal today isn’t to tell people what to do,” he said. “My goal is to urge our fellow citizens to participate in a conversation as a country about where we are, and where we want to be, with respect to the authority of law enforcement.”

Listing image by FBI