Footage from surveillance cameras along the Boston Marathon route gave the FBI early clues about the bombing suspects. And prosecutors say they'll use some of those images to try to prove their criminal case against 19-year-old Dzhokhar Tsarnaev. But the proliferation of cameras in America's big cities is raising some tricky questions about the balance between security and privacy.

It was pictures of two brothers taken by a camera outside the Lord & Taylor department store that provided the first glimpse of the men who allegedly bombed the Boston Marathon.

"What was perfectly obvious after the attack was the most important thing was to know who had attacked us, and to begin the process of tracking them down," says former Homeland Security official Stewart Baker, who says he's been watching the events unfold. "And the cameras were absolutely essential for that purpose."

Government Cameras Vs. Private Cameras

Baker says even though some amateur sleuths misidentified the men in those images, he expects to see more cameras playing a role in big investigations — not to deter crime, but to catch criminals after the fact.

He points out that similar images helped authorities in London get to the bottom of who bombed the subway system there back in 2005.

But privacy expert Julian Sanchez, a research fellow at the Cato Institute, says there was one critical difference there — those closed-circuit TV cameras throughout London are operated by the government.

Boston has only about 60 cameras controlled by law enforcement, Sanchez says. The cameras that proved so helpful there last week were privately owned.

"In a way, private, distributed surveillance cameras create a kind of network of 'little brothers' instead of a Big Brother, so you get the same benefits to a great extent without incurring the risks to civil liberties," he says.

That's because those cameras owned by local businesses target small areas. They capture activity that most people in that public place can see with their own eyes. And they usually record over themselves every week, cutting down on the intrusions to privacy.

One more thing: Those company-owned cameras generally don't feed back to a big networked system, where police are watching.

Surveillance Limits?

Ben Wizner, who directs the speech, privacy and technology project at the American Civil Liberties Union, says he doesn't object to the way police used surveillance in Boston.

"I think, in some ways, this is an easy case because when there's a crime of this nature, there's no problem whatsoever for the police to get any kind of permission they need from judges in order to conduct surveillance," Wizner says.

Wizner says that means going to a judge to get a warrant for the images on privately owned cameras or taking advantage of an emergency exception in the law to get that footage more quickly. But he warns that surveillance can go too far.

"The questions that we have are: Do we want a society in which cameras are literally everywhere and we can't walk down the street holding someone's hand without being recorded in a government database? And then, what happens to all of this personal footage, almost all of which does not capture terrorists, when the event has been solved?" he adds.

The ACLU says it wants authorities to be careful about storing those images of innocent people in law enforcement databases with no time limit for erasing them.

Rep. Adam Schiff, D-Calif., a member of the House Intelligence Committee and a former federal prosecutor, says the tug of war over privacy is nothing new.

"It goes back to the Constitution," Schiff says. "It was part and parcel of the Fourth Amendment right to be free of unreasonable searches and seizures. We've always expressed a strong constitutional preference for privacy, for not giving the government unbridled authority, even to protect us."

Schiff says that means we sometimes take risks, but that's all part of living in a free society.

Copyright NPR 2017.