Last Friday a federal judge in Kansas turned away a civil rights lawsuit by a Leawood couple whose house was searched in 2012 based on a visit to a hydroponics store and a field test that incorrectly identified tea in their garbage as marijuana. In a summary judgment, U.S. District Judge John Lungstrum said police acted legally and reasonably in planning and conducting the fruitless raid on the home of Robert and Adlynn Harte, former CIA employees whose children were 7 and 13 at the time. The family was held at gunpoint for two and a half hours while Johnson County sheriff's deputies went through the house, after which they gave the Hartes a receipt saying "no items taken" in lieu of an apology. Because the cops refused to say why they thought the Hartes were growing marijuana, the couple spent a year and $25,000 in legal fees to get a look at the affidavit supporting the search warrant.

Among other things, Lungstrum's ruling means he thought the evidence cited in that affidavit provided probable cause for the search. If so, that's only because probable cause is a much weaker standard than people generally imagine.

It turned out that the genesis of the search was a tip from a Missouri state trooper who saw Robert Harte leave a Kansas City hydroponics store on August 9, 2011, carrying a bag. Inside the bag were supplies for a horticultural project involving tomato, squash, and melon plants that Harte thought would be edifying for the kids. Since people often buy indoor gardening supplies for such perfectly legal purposes, that purchase itself was not enough for probable cause. But eight months later, sheriff's deputies rummaging through the Hartes' trash came across wet "plant material" that the Hartes think must have been some of the loose tea that Adlynn favors. Although a field test supposedly identified the material as marijuana, a laboratory test (conducted after the raid) showed that result was erroneous.

The Hartes argued that police should have known better than to trust field tests, which are notoriously inaccurate. Experiments by Claflin University biotechnologist Omar Bagasra found that one commonly used field test, the NarcoPouch KN Reagent, misidentified many legal plant products as marijuana, including spearmint, peppermint, basil, oregano, patchouli, vanilla, cinnamon leaf, lemon grass, bergamot, lavender, ginseng, anise, gingko, eucalyptus, rose, cloves, ginger, frankincense, vine flower, chicory flower, olive flower, cypress, and St. John's wort. Several of those are common ingredients in herbal tea. The Hartes say the test used to incriminate them has a "false positive rate" of "up to 70 percent on common kitchen botanicals." They also note that the test is not supposed to be performed on "saturated or liquid samples."

But according to Judge Lungstrum, the innocent act of visiting a hydroponics store, combined with the results of a test that misidentifies many common organic substances as marijuana even when it is performed correctly, adds up to probable cause for a search. Which makes you wonder: What the hell is probable cause? The phrase, which the Fourth Amendment prescribes as the basis for a search warrant, has never been precisely defined, but the Supreme Court has said it need amount to no more than a "substantial chance" or a "fair probability" that evidence of a crime will be discovered. It's clear that a probability substantially lower than 50 percent will do, which is why courts continue to treat highly unreliable indicators such as drug field tests and dog alerts as sufficient to justify a search.

A couple of good things did result from the police invasion of the Hartes' home. It inspired them to lobby for a change to Kansas law that made probable cause affidavits presumptively a matter of public record, as they are in other states. The Kansas City Star reports that "their lawsuit also prompted the Johnson County sheriff's office to now require lab confirmation of suspected drug material."

[This post has been revised to clarify the nature of the "false positive rate" cited by the Hartes.]