Fruit of the Poisonous Tree

What the doctrine means

The exclusionary rule bars evidence obtained through an unconstitutional search or seizure. If that were all, police could use an illegal search to learn where to look and then collect the “real” evidence later. The fruit of the poisonous tree doctrine closes that gap: the unlawful search is the tree, and evidence derived from it can be excluded as its fruit. In Wong Sun, that included both physical evidence and statements.

The test: exploitation, not just “but for”

The leading case is Wong Sun v. United States (1963). Federal narcotics agents entered James Wah Toy’s laundry and arrested him without probable cause. His statements led them to another man, Johnny Yee, who surrendered heroin. The Court held the drugs couldn’t be used against Toy because they were “come at by the exploitation of that illegality.”

The Court also limited the doctrine. Evidence isn’t fruit of the poisonous tree “simply because it would not have come to light but for the illegal actions of the police.” The question is “whether, granting establishment of the primary illegality, the evidence to which instant objection is made has been come at by exploitation of that illegality or instead by means sufficiently distinguishable to be purged of the primary taint.”

How a chain can form in an ALPR case

In a case involving a license plate reader, the camera record is often the first link rather than the evidence that matters most at trial. A typical sequence might look like this:

1. A camera reads a plate and generates a hotlist alert.
2. Officers stop the car.
3. During the stop, they question the driver, see something in plain view, or search the car.
4. What they find leads to further searches, statements or warrants.

If the stop at step 2 was unlawful, for example because officers acted on an alert they hadn’t verified and lacked reasonable suspicion, the defense may argue that what came after was obtained by exploiting it. The same reasoning can apply to a search of stored ALPR history that located a suspect, if that search is found to have been unlawful.

The camera record itself is usually not the target. Evidence of where a vehicle was seen by a fixed camera generally exists independently of the stop. What a successful challenge most often affects is what officers found because of the stop or search.

Where the chain breaks

Courts don’t follow the chain forever. Three doctrines can cut it off. Each is explained in exceptions to the exclusionary rule.

• Independent source: the evidence was also obtained through a genuinely independent lawful route (Murray v. United States).
• Inevitable discovery: the prosecution shows by a preponderance of the evidence that lawful means would have found it anyway (Nix v. Williams).
• Attenuation: the link is too remote, judged by time, intervening circumstances, and especially the purpose and flagrancy of the misconduct (Brown v. Illinois). In Utah v. Strieff, discovering a valid, pre-existing arrest warrant during an unlawful stop was enough to break the chain.

A separate threshold applies too: the person seeking suppression generally must show their own rights were violated. See standing.

Questions a defense attorney may investigate

• What was the first step in the investigation: an alert, a database search, or something else?
• Was that first step lawful, and on what facts?
• What evidence was found after it, and how did officers get from one step to the next?
• Is there any independent source for that evidence, or a claim it would have been found anyway?
• Did anything intervene, such as an outstanding warrant, between the violation and the discovery?