People v. Jennings: The Case That Brought Fingerprints to U.S. Courts

People v. Jennings, decided by the Illinois Supreme Court in 1911, was the first ruling by an American appellate court that fingerprint evidence could be used to identify a defendant at a criminal trial. The court affirmed the murder conviction and death sentence of Thomas Jennings, holding that expert comparison of fingerprints rested on a scientific basis and belonged in the courtroom. That single decision opened the door to more than a century of forensic identification in American criminal cases.

The Chicago Murder Behind the Case

On the night of September 19, 1910, an intruder broke into the Chicago home of Clarence Hiller. Hiller confronted the man, and during the struggle he was shot and killed. Officers who searched the scene noticed something unusual on a back porch railing: four clear fingerprint impressions from a left hand, pressed into paint that had been applied only hours earlier and was still wet.¹

Police stopped Thomas Jennings a short distance from the house not long after the shooting. His clothing was torn and bloodstained, and he was carrying a loaded revolver later linked to the bullets that killed Hiller. Jennings had recently been released from the state prison in Joliet after serving time for burglary, and his fingerprints were already on file.¹

The Evidence Presented at Trial

Investigators removed the section of railing bearing the impressions and photographed the prints. Prosecutors displayed enlarged photographs at trial alongside ink prints taken from Jennings’s prison records. Four fingerprint examiners testified. Two were Chicago police officers with fingerprint experience, one was an inspector from the Dominion Police in Ottawa, and one was a U.S. government investigator trained at Scotland Yard, the first police force in the world to adopt fingerprint analysis for detective work. All four independently concluded that the prints on the railing and the prints on file came from the same hand.¹

By 1910 fingerprints were already used in the United States for prison records and federal identification, and the classification method behind the analysis had been in use at Scotland Yard since 1901. Using them as courtroom proof in a murder case, though, was new.

The Novel Legal Question on Appeal

Jennings was convicted and sentenced to death. On appeal to the Illinois Supreme Court, he argued that the fingerprint evidence should never have been admitted. The question was clean and unprecedented: could fingerprint comparison be used to prove a defendant’s identity in court? No American appellate court had answered it, and no statute authorized the practice. Chief Justice Orrin Carter wrote that the court could find “no case in which this question has been raised” and “no statutes or decisions touching the point in this country.”¹

What the Court Held

In People v. Jennings, 252 Ill. 534 (1911), the Illinois Supreme Court affirmed the conviction. Chief Justice Carter concluded that “there is a scientific basis for the system of finger-print identification and that the courts are justified in admitting this class of evidence.”¹

The reasoning rested on a few points. The four experts brought real credentials and reached the same conclusion independently. They explained that ridge patterns are unique to each individual and remain permanent throughout life, giving the technique observable and classifiable features rather than guesswork. And although the method was untested in American courts, it already had a track record with Scotland Yard, police forces in British colonies, and various government identification bureaus. The court also treated fingerprint comparison as a proper subject for expert testimony because ridge analysis fell outside the ordinary experience of jurors. Without trained examiners to explain the comparison, the evidence would have carried no meaning.

Thomas Jennings was executed by hanging on February 16, 1912. He is generally considered the first person in American history convicted on the strength of fingerprint evidence.

Why the Case Still Matters

Jennings gave fingerprint evidence formal legal legitimacy in the United States. Courts across the country cited the ruling as precedent, and within a generation fingerprint identification had become routine in criminal investigations and prosecutions.

The larger contribution was a framework. Jennings showed that courts could evaluate emerging science, decide whether it was reliable enough to reach a jury, and admit it under expert testimony when the answer was yes. That approach shaped how American courts would later handle ballistics, toxicology, questioned documents, and eventually DNA.

How Admissibility Standards Changed After Jennings

When Jennings was decided, no formal legal test existed for scientific evidence. The court asked, in essence, whether the technique had a scientific basis and had gained acceptance among practitioners. Twelve years later, in Frye v. United States (1923), a federal appeals court turned that instinct into a rule: scientific evidence was admissible only if it had gained “general acceptance” in the relevant scientific community. Frye governed forensic evidence in most American courts for seventy years.

In 1993, the U.S. Supreme Court replaced Frye in federal courts with a more flexible reliability inquiry in Daubert v. Merrell Dow Pharmaceuticals. Under Daubert, trial judges act as gatekeepers, weighing whether a method has been tested, whether it has been peer-reviewed, its known error rate, whether standards govern its use, and whether the scientific community broadly accepts it. Congress later codified that gatekeeping role in Federal Rule of Evidence 702, which requires expert testimony to be based on sufficient facts, produced by reliable methods, and reliably applied to the case at hand.²

Fingerprint evidence has survived challenges under both Frye and Daubert. What has changed since 1911 is the level of proof courts expect. The deference the Jennings court showed to expert confidence and international practice would not, on its own, satisfy a modern reliability inquiry.

How Fingerprint Evidence Has Held Up

For most of the twentieth century, fingerprint analysis was treated as effectively infallible. That assumption began to break down in 2009, when the National Academy of Sciences published a wide-ranging review of forensic science. The report found a “dearth of peer-reviewed, published studies establishing the scientific bases and reliability of many forensic methods,” fingerprints included. It criticized inconsistent terminology across the field, noting that words like “match” and “consistent with” were not clearly defined, and it called claims of “zero-error rates” implausible. Examiners, the report observed, did not always reach the same conclusions when shown the same evidence in a different context.³

In 2016, the President’s Council of Advisors on Science and Technology attempted to put numbers on the problem. Reviewing controlled studies of examiner performance, PCAST reported that an FBI-affiliated study found a false-positive rate with an upper bound of roughly 1 error in 306 examinations, while a study conducted by the Miami-Dade crime laboratory found an upper bound of 1 error in 18.⁴

Neither report led courts to exclude fingerprint evidence, but both changed the conversation. Modern examiners typically follow a structured protocol known as ACE-V (analysis, comparison, evaluation, and verification), in which a second qualified examiner independently repeats the work before a conclusion is reported.⁵ Critics point out that the underlying comparison still rests on human judgment rather than quantitative measurement.

The Jennings court described fingerprint identification as resting on a scientific basis, and no American court has reversed that conclusion. What has shifted in the century since is candor about the method’s limits. Jennings settled the admissibility question. Later courts, and later science, are still working out how to communicate the uncertainty around it to juries.

1
State of Illinois Office of the Illinois Courts. Illinois Supreme Court History: Fingerprints
2
Legal Information Institute (Cornell Law School). Rule 702 – Testimony by Expert Witnesses
3
National Academies of Sciences, Engineering, and Medicine. ‘Badly Fragmented’ Forensic Science System Needs Overhaul; Evidence to Support Reliability of Many Techniques is Lacking
4
President’s Council of Advisors on Science and Technology (PCAST). Forensic Science in Criminal Courts: Ensuring Scientific Validity of Feature-Comparison Methods
5
National Institute of Standards and Technology (NIST). Standard for the Documentation of Analysis, Comparison, Evaluation, and Verification (ACE-V) (Latent)