Last month’s tragic attack on the Boston marathon leaves us wanting answers — not just about why it occurred, but why we failed to prevent it. One tempting answer is that the FBI could have prevented the Boston attack if it had more power and fewer legal encumbrances. That seems to be the wrongheaded if understandable impulse of former Sen. Joe Lieberman (I-Conn.), who at last Thursday’s House Homeland Security Committee hearing on the Boston attack, urged Congress to review the Attorney General Guidelines that regulate the FBI’s surveillance and investigation power.

In our democratic society, a thought crime is no crime at all. Yet Lieberman and some members of Congress suggested that the FBI should be able to keep investigations open based on a person’s religious and political beliefs. That change would be ruinous to an agency that prides itself on upholding the Constitution, and it would not help prevent terrorism.

Since 2008, the Attorney General Guidelines have permitted the FBI to survey what YouTube videos you watch and eavesdrop on your subway platform conversations, without suspecting you of planning or committing a crime. There is no time limit on these investigations, which the FBI calls assessments, but under the rules they cannot be initiated or justified based only on a person’s religion, race or political beliefs.

There is evidence that the FBI has nevertheless conducted wide-scale surveillance, deploying informants to community centers and places of worship to eavesdrop and spy on Muslims, environmental activists and political dissenters of various stripes. The FBI keeps about 15,000 informants on its payroll. Within two years of the period in 2011 when it investigated Tamerlan Tsarnaev, it opened more than 80,000 assessments.

In important ways, though, the FBI has resisted becoming a domestic spy agency. It has distanced itself from the New York Police Department’s abusive program of blanket surveillance of mosques and Muslim-owned businesses. In some cities, the FBI has sought to build cooperative relationships with local Muslim communities, preferring reliable tips from these communities to fishing expeditions. Although the FBI has sometimes wrongly used its community outreach efforts to gather intelligence, many FBI agents recognize the importance of establishing genuinely rights-respecting community partnerships.

Loosening the Attorney General guidelines and asking the FBI to prowl for would-be terrorists indefinitely and indiscriminately would be a step backward. It would crush the FBI with an avalanche of useless data and false leads — imagine if all 80,000 assessments the FBI conducted, not just Tsarnaev’s, had been kept open and added to every year. It would undermine the agency’s sense of fidelity to the Constitution and First Amendment freedoms, charging it to disregard legal limits it has regarded as sacrosanct.

Expanding the FBI’s powers is also unlikely to prevent the next Boston-style attack. Lieberman, and policymakers who likewise subscribe to the theory that “radicalization” to terrorism can be predicted based on religious behavior or political expression, give too much credit to terrorism, treating it as rationally calculated to achieve a political end or fulfill a religious purpose, even when, as with Tsarnaev, there was reportedly no link to a terrorist network or movement.

Thousands of Americans browse YouTube videos and Web chat rooms for information on radical ideas. Some hail from American Muslim communities while others are anti-tax libertarians or stalwart environmentalists. Very few seriously contemplate committing violence.

The hard truth is that there are no easy predictors for the senseless and terrifying acts of violence the country has recently seen — from Boston to the mass gun violence in Aurora, Colo., and Newtown, Conn. At play in each was a toxic cocktail of alienation, isolation and mental dysfunction. It is a mistake to view the perpetrators — all disaffected young men — as acting rationally enough that the FBI, with more blanket sleuthing and surveillance, could have smoked them out. Government support for counseling and psychological services, social work and youth programs is not the obvious answer to terrorism. But as allies like the U.K. have found, it can yield local, community-driven solutions to the social fragmentation that contributes to terrorism, and it is more likely to be effective than national-level programs that propose quick, oversimplified answers.

Where ideology plays a larger role in terrorism, the best prevention is counter-ideas. Perversely, fear of FBI surveillance has made some American Muslim leaders feel they must shun or silence, rather than engage with, youth who show interest in violent extremism. Less surveillance, not more, is likely to free these communities to address violent extremist beliefs with anti-violence principles of Islam and democratic values already abundant within them.

Shah and Ismail are researchers at Columbia Law School’s Human Rights Institute, where they have conducted extensive study on domestic counterterrorism investigations and their impact on American Muslim communities.