Which is more intrusive: security screening and metal detectors every few blocks, or a drone flying high above it taking video of every little thing you do?

"The best thing would have been a dog," explains Joseph King, professor of terrorism and organized crime at John Jay College of Criminal Justice and former chief of counterterrorism for U.S. Customs and Border Protection. "They don't need to be at a choke point; they can move through the crowd."

In the wake of the tragedy at the Boston Marathon on April 15 security experts are pondering the use of bomb-sniffing dogs and other tools of the security trade during public events, at the risk of making those events as much fun as catching a flight. Ultimately, the costs of such extra measures—both in terms of money and loss of privacy—would have to be weighed against their actual ability to prevent tragedies.

The complicating factor in making public events like the Boston Marathon safe is their sheer size. Scanning equipment, checkpoints and other technology already familiar from airport screening would not have done much to prevent this marathon attack; instead they merely displace the threat to where the queue for that security screening masses people. As a RAND Corp. analysis of security at Los Angeles International Airport showed, reducing the time spent in ticket lines and security processing was the single most important variable in reducing the potential number of casualties and injuries from this type of terrorist attack.

Video footage is another common tool to identify criminals. Some authorities suggest that increased camera surveillance—of the kind familiar to European countries that helped capture the perpetrators of the London bombings on July 7, 2005—might be another way to enhance security. "Cameras have to be a fact of life in the U.S., which people don't like," King argues. But "you have no right to privacy walking down Third Avenue."

Although such video footage is useful in catching suspects after the fact, using them as a preventative tool is far harder—except for whatever deterrent effect such conspicuous technology may exert. Human eyes can only see so much, and analytic software has not advanced to the point where it can detect a threat, although such software does now know enough to cue a human operator to pay particular attention to what's happening on one monitor versus another. But instead of installing expensive camera and monitoring equipment, the Boston bombing and its aftermath may point the way to crowd-sourcing that security function. Authorities have called for all those who have photos or videos of the event from cellphone cameras to submit them. "There is this crowd-sourced camera system that covers a lot of everything, most of the time," says terrorism expert Brian Jackson, director of RAND's Safety and Justice Program. "It's less expensive to rely on a camera that exists,” in the form of an iPhone, “than to put in a camera system."

The remotely operated mini airplanes known as drones may also prove a low-cost option for increasing surveillance of such events. Advances in technology might one day make such drones capable of detecting low concentrations of explosives from high altitude, though that remains a distant prospect. "You can fly a drone for $300 a year instead of a helicopter that costs $300 million," King notes.

At the same time, police officers have become not just first responders but first preventers of such terrorist attacks. For example, two students stopped by police for a traffic violation in South Carolina were subsequently found to have explosives in their vehicles. Similarly, state troopers and undercover cops have captured would-be terrorists after noting "suspicious activity," yet such suspicions proved ill-founded in the follow-up to the Boston bombing. "Low-end threats can do a lot of damage and they're not 100 percent preventable in any way," Jackson says. But if the U.S. can "keep terrorists from being able to do anything but first attacks over the long term, we win."

Outside of surveillance, authorities would find it extremely difficult to prevent someone from gathering the materials needed to put together a bomb of the sort used in the Boston attack, which involved a household pressure cooker, along with other readily available components. "We put Sudafed behind the counter and you need a driver's license to get it, but that doesn't make sense to do with a pressure cooker," Jackson notes.

And securing an entire 26-mile-long route of a marathon would be impossible without an army of tens of thousands of security personnel. Such a long route is "the definition of an indefensible perimeter," Jackson says. The London Marathon on April 21 has "reviewed and will continue to review our security arrangements," according to a press release, but the cost of any extra security measures may prove prohibitive for other races and events.

Ultimately, societies will have to examine the tradeoff between civil liberties and public safety, risk and security versus the freedom to hold events such as the Boston Marathon. The risk of terrorist attack remains vanishingly small in the U.S.—more Americans die in a single month from heart disease or car crashes than have ever been killed or wounded by terrorists. "Even if you have a police state you can't secure every inch of every foot for every minute," King notes. "Professionally, I would say don't change things, but that's not a popular place for a police commissioner to be."