If someone develops a practical mind-reading device, you can expect the Department of Homeland Security to argue that skulls are merely another "closed container" that officers guarding the border may search at will. After all, government agents have long been allowed to read documents in briefcases carried by Americans returning from abroad. Why should the medium in which information is stored make a constitutional difference?

That argument is only slightly more far-fetched than the one DHS uses to justify its policy regarding border searches of laptop computers. Given the nature and quantity of the data they contain, portable computers are in many ways extensions of our brains. Yet DHS is treating them as if they were no different from purses or fruitcake tins.

Recently publicized DHS guidelines confirm that the department for years has been examining the contents of computers at airports and other points of entry "absent individualized suspicion." The guidelines say officers "may detain documents and electronic devices, or copies thereof, for a reasonable period of time to perform a thorough border search," which "may take place on-site or at an off-site location."

In practice, this means a customs agent can seize your computer for any reason or no reason at all. He may rummage through your files while you stand there, hoping nothing embarrassing pops up, or he may take the computer to a back room. It may disappear for weeks or months as its contents are copied, analyzed and shared with various federal agencies trying to determine whether you've broken any laws.

DHS hasn't said exactly how common these searches are. But when the Association of Corporate Travel Executives surveyed its members in February, 7 percent of the respondents said their laptops or other electronic devices had been seized.

As anyone whose computer has been stolen or irreparably damaged can testify, it's a traumatic experience to suddenly lose this crucial repository of your personal and professional life, which may include confidential work in progress; sensitive financial, medical and educational records; and years of photos, music, notes, journal entries and correspondence. Knowing that government agents are perusing and passing around this information makes the experience even less pleasant, especially when you realize that your hard drive also contains traces of files you've deleted and websites you've visited.

But according to DHS, all that's necessary to make this enormously inconvenient and invasive search "reasonable" under the Fourth Amendment is your decision to take your computer with you to another country. In April, the U.S. Court of Appeals for the 9th Circuit agreed, reversing a lower court decision that said such searches require "reasonable suspicion," a belief based on "specific and articulable facts," along with "rational inferences" drawn from them.

This standard, substantially less demanding than the "probable cause" necessary for a warrant, is not very hard to satisfy. In fact, DHS Secretary Michael Chertoff claims the department already follows it. "As a practical matter, travelers only go to secondary [examination] when there is some level of suspicion," he writes in a July 16 USA Today op-ed piece. But he warns that "legislation locking in a particular standard would have a dangerous, chilling effect as officers' often split-second assessments are second-guessed."

It's worth emphasizing that Chertoff is talking about searches for incriminating data, not searches for bombs or other imminent threats. He says computer searches have turned up "violent jihadist material" as well as "scores of instances of child pornography." He does not say how many, if any, terrorist plots have been foiled. As electronic privacy expert Peter Swire points out, even a "moderately well-informed" terrorist can easily avoid detection by emailing encrypted data to himself or using software that hides files.

Judging from Chertoff's statements and the legal record, the government is using fear of terrorism to justify extraordinarily invasive, suspicionless searches in the service of ordinary police work. In these circumstances, the real danger is the absence of second-guessing.