Illinois police officers often use drug-sniffing dogs to provide probable cause for warrantless vehicle searches during routine traffic stops, but statistics show that these dogs may be surprisingly unreliable.

In what is known as an “alert,” drug-sniffing dogs are trained to sit or dig when they detect certain scents during a traffic stop, sending a signal to officers that illegal drugs may be present and triggering a legal search of the vehicle on the spot. However, in an analysis of one K-9 unit conducted by Huffington Post, only about a quarter of the alerts from one drug-sniffing dog resulted in officers finding measurable quantities of illegal drugs. The analysis was based on Illinois State Police records from an 11-month period in 2007 and 2008.

According to the report, the dog conducted 252 “sniffs” and alerted 136 times during that time period. Just 35 of those alerts led to the discovery of drugs in a large enough quantity to merit an arrest, while 63 turned up only residue. Significantly, according to the Huffington Post article, what officers identified as “drug residue” in those cases was not subjected to lab testing, and in some cases is likely to have been something else entirely.

In 38 cases, or nearly 28 percent of the alerts in the Huffington Post analysis, officers found no drugs or residue at all. A similar investigation conducted by the Chicago Tribune in 2011 revealed a similarly high error rate. According to the Tribune’s analysis of data from suburban Illinois police departments over a three-year period from 2007 to 2009, officers discovered illegal drugs or drug paraphernalia just 44 percent of the time after a dog alerted, and only 27 percent of the time in alerts involving Hispanic drivers.

There are various reasons why a police dog may alert even when there are no drugs present in a vehicle. For instance, a police dog’s sense of smell may be sensitive enough to detect the scent of substances that are no longer in the vehicle, such as drugs belonging to a passenger or former owner of the car. Although the dogs can be trained to alert only to non-residual quantities of drugs, both the Tribune and the Huffington Post article suggest that inadequate training may be a factor in the high error rate. In addition, a dog expert quoted by the Tribune believes that in some cases the dogs may be reacting to subtle queues given off – intentionally or unintentionally – by their handlers.

If you have been arrested following a police stop of your vehicle, contact an experienced criminal defense attorney for legal assistance.