Shortly after the Boston Marathon bombings, President Obama described the work being done by the FBI and Department of Homeland Security to unravel the plot as "hard stuff."
But judging by the uproar sparked by Wednesday's revelation that the White House had sought a secret order for Verizon to turn over phone records on its customers to the National Security Agency, determining where to draw the line between an individual's privacy and homeland security could well prove to be the hardest of the hard stuff.
That line is subject to tectonic shifts influenced not only by events, but also by public attitudes, a changing legal framework and technological advances that have given investigators ever-more ability to peer into our private comings and goings, says Daniel Byman, a counterterrorism and security expert at the Brookings Institution.
Byman, who says he has no special knowledge of what the goal of acquiring the phone records would be, says it could nonetheless give the NSA and FBI "the raw data to map a network of terrorist activity."
"With modern computing power, they have the ability to sift a large mountain of data to distill a significant bit of refined data," he says. "What I can imagine happening is for millions of telephone numbers to be refined to, say, a few dozen. A cellphone number that's attached to a suspected militant might lead to other suspects simply by virtue of who's calling whom."
Kent Greenfield, a professor of constitutional law at Boston College, believes that the government's request, which falls under Section 215 of the Patriot Act, looks like a "very broad and aggressive interpretation" of that provision.
"I think it's evidence of surveillance creep," Greenfield says. "We are becoming more and more used to having our data surveilled for public and private activities. ... It's hard for any individual to know if their data is swept up in it."
But in a post-9/11 era of nearly complete information-sharing via social media and electronic commerce, does the public care that the government knows who you talked to (but, apparently not what you said), especially if handing that information over might reduce the risk of a terrorist attack?
A look at two polls conducted by The Washington Post shows the Catch-22 faced by any president when it comes to balancing these concerns:
In January 2010, 63 percent of Americans surveyed believed that the Obama administration would "not go far enough" to investigate terrorism because of concerns about constitutional rights, while just 27 percent thought the president would go too far. In a similar poll conducted by the Post on April 18, just days after the Boston bombings, barely 40 percent thought the president would not go far enough to investigate terrorism, while nearly half feared he would go too far.
"There's a political element to this," says Byman. "The civilian liberties community, mainly on the left, took the Bush administration to task over this sort of perceived overreach. But now the libertarian right, which was largely silent then, has joined in the criticism."
Matthew Dallek, a historian and public policy scholar at the Wilson Center, agrees that it is difficult for the White House to strike the right balance.
"I think we have this vast intelligence bureaucracy and it's on the firing line," he says. "If those guys get it wrong, they're going to get the blame.
"The president and everyone else in his sphere believe that protecting Americans is their greatest responsibility," he says. "By comparison, civil liberties usually take a back seat."
There's also a disconnect between how the public perceives the government's collection of potentially sensitive data and how we view a corporation doing the same thing.
"As we are speaking, I have Google up on my computer," says Byman. "I have no doubt that Google knows more about my electronic activities than does the government. But there seems to be this greater acceptance of that than of the government having the same information. Interestingly, in Europe, it's the opposite, they are less concerned there about the government and more concerned about corporations."
Although the Internet has accelerated the ability of intelligence agencies such as the NSA to collect data on individuals at the same time that the threat of terrorism has changed attitudes about privacy, it's not a new issue, Dallek says.
"In World War II, there was a lot of concern about so-called fifth columnists," he says, referring to a term used to describe people who were believed to be secret fascist sympathizers intent on attacking the U.S. from within.
"It was not uncommon for ordinary Americans to write to their government and ask, for example, that a German-speaking neighbor be monitored for possible subversive activity," he says. "The FBI under J. Edgar Hoover took that sort of thing very seriously."
Dallek says that in the 1930s, well before the wartime internment of Japanese-Americans, President Roosevelt wrote a memo calling for the surveillance of Hawaiians of Japanese decent over fears they could be relaying intelligence on the U.S. Navy base at Pearl Harbor to Tokyo.