Time will tell whether the decision to arrest Mr. Zazi and his associates was premature. If the case against them does not develop beyond what has been reported, and if no useable evidence is developed against the 24 other men, the decision to arrest will be second-guessed. That would be grossly unfair. From a public safety perspective, law enforcement officers and prosecutors cannot be faulted for acting when they believe that the public is in imminent peril, even if that means compromising an investigation.

The larger issue raised here is whether there is a viable alternative to the nerve-racking game of chicken that law enforcement must play in terrorism cases. The obvious — though extremely unpopular — alternative is the passage of a preventive detention statute.

Such statutes have been upheld in the context of people with a demonstrated proclivity toward violent conduct, like sexual predators; the concept could be adapted, in a way that withstands constitutional scrutiny, to cover people with a demonstrated proclivity toward terrorism. That approach would give law enforcement additional means to disrupt potential terrorist plots. It has the virtue of honesty, obviating the strained and sometimes disingenuous use of material-witness and false-statement statutes that are now frequently used to arrest and hold suspected terrorists, and would remove the temptation to criminalize conduct that borders on free speech.