In March 1932, aviator Charles Lindbergh’s 20-month-old son was kidnapped from his nursery near Hopewell, New Jersey, and held for ransom. In May Charles Lindbergh Jr.’s body was discovered. A German carpenter named Bruno Richard Hauptmann was convicted of the crime—based in part on the handwriting analysis and testimony of Wilmer Souder, a publicity-shy physicist working in the fledgling field of forensic science.

Souder, PhD 1916, kept his involvement with the “trial of the century” secret. He kept such a low profile that near the end of his career, he was dubbed Detective X by Reader’s Digest. His crime-fighting legacy was all but forgotten until nine dusty green notebooks surfaced in 2014. They told the story—in Souder’s nearly illegible handwriting—of how he became a forebear of federal forensics.

Born in Salem, Indiana, in 1884, Souder attended Indiana University before joining the National Bureau of Standards, a federal agency established in 1901 to set standards for scientific and industrial measurements. After two years, Souder left to earn his PhD at the University of Chicago, studying physics under Nobelists Albert A. Michelson and Robert Millikan. Souder focused on the photoelectric effect—when a material emits electrons as light shines on it. (Millikan’s 1923 Nobel Prize was based partly on the research Souder conducted.) This work gave Souder a grounding in the art and science of precise measurements.

Eventually Souder returned to the bureau to work in its division of metrology, the scientific study of measurements. His focus was thermal expansion, how certain substances change under different temperatures—for example, a dental filling in contact with ice cream or hot soup. In 1919 Souder established the Department of Dental Research to standardize dental materials. This became the work he is best known for, with a prestigious dental research award named in his honor.

Less well known is the physicist’s side gig: helping to convict gangsters, tax evaders, and murderers.

America’s first federal crime lab was housed at the National Bureau of Standards, which handled forensic analysis on hundreds of cases for government agencies. Souder was the driving force behind its foray into forensics, yet his identification lab was but a footnote in the bureau’s history—a literal footnote in Measures for Progress: A History of the National Bureau of Standards (US Department of Commerce, 1966).

That changed in 2014, when physicist Kristen Frederick-Frost, a museum curator at the bureau’s successor agency, the National Institute of Standards and Technology, was designing an exhibition on forensic science history. She uncovered a meticulous—though incomplete—record of Detective X’s investigations in a box of notebooks dating back to 1929. Frederick-Frost teamed up with analytical chemist John Butler, a leader of the institute’s efforts to strengthen the scientific validity of forensic science. Their research revealed Souder as a rigorous scientist intent on legitimizing the emerging field of forensics through precision and circumspection.

Souder’s interest in applying science to crime likely began in 1913, when Albert Osborn, one of the earliest document examiners, sent micrometers (precision measurement instruments used with microscopes) to the bureau for calibration. Souder, who had tested the devices, eventually followed Osborn into the world of document analysis, studying how handwriting, typewriter models, and ballpoint pens left unique marks. With a close eye, scientists could, in theory, tell a forgery from the real deal or connect a note to its writer.

Souder also established ballistics research in his lab, matching ammunition to the gun that fired it based on markings etched on the bullet. Using a recently invented comparison microscope, which allowed an examiner to view two objects side by side at the same magnification, Souder could match up scratches and grooves in minute detail.

While Souder was analyzing evidence for criminal cases, he was developing standards for methodology and interpretation, the purview of his agency. Adding measurement to observation helped support evidence with data, which could be compared case to case and used to assess an expert’s qualifications—thus standardizing forensic science.

Between 1929 (well into his criminology career) and his retirement in 1954, Souder worked on 838 cases, almost all for the federal government. Nearly 300 cases were for the Treasury Department, which famously investigated gangsters for tax evasion.

The Lindbergh kidnapping-turned-murder investigation was an exception. The New Jersey State Police approached Souder soon after the baby’s disappearance in the spring of 1932, with more than a dozen ransom notes to analyze. Souder concluded that all had been written by the same person.

In his records, Souder coded the Lindbergh case as the Adamson case and gave his police department contact an alias. (The actual Adamson/Lindbergh notebook has yet to be found.) As with other casework, he eschewed the spotlight to protect himself and his family. Despite his best efforts, the Chicago Tribune somehow discovered his identity and leaked his involvement in the Lindbergh case. In a box of family memorabilia Souder’s granddaughter offered the institute’s museum was a gun-carry permit stating, “Witness in criminal identifications for Federal & D.C. Governments. Personal protection desirable for such service.”

The Lindbergh baby’s remains were accidentally found about a month after the family paid the ransom with marked bills. Lindbergh’s father-in-law was a US senator and therefore the pool of potential suspects vast. Souder analyzed more than 8,000 documents from the Senate office building in less than three weeks (it’s unclear whether he had help), but no matches were made.

Following a nationwide search, Hauptmann was caught in New York passing the marked bills and put on trial. Eight handwriting experts, including Souder and Osborn, testified that samples of Hauptmann’s writing matched the ransom notes. He was convicted and sentenced to death.

Hauptmann’s guilt has long been questioned, yet handwriting analysis was just one of several pieces of circumstantial evidence. Many clues were obtained through old-fashioned (nonscientific) detective work, including artist sketches, the discovery of marked bills in his possession, and the conclusion that wood from the homemade ladder used to reach the nursery window matched the wood of Hauptmann’s attic floor. Even though traditional police work largely won the day, the public’s imagination had already been captured by the possibilities of forensic science. A newspaper article from 1929 that covered Souder’s work announced, “Scientific Sherlock to Supplant Gumshoe Detective.”

The high-profile Lindbergh case spurred New Jersey to establish its own state crime lab, and the FBI founded its crime lab in 1932. Souder recommended scientific instruments and taught lab personnel. In 1940 the US Postal Service founded its forensics lab, with Souder training its document examiners. He also wrote about forensics research for trade and agency publications to help other investigators learn his methods.

Since Souder began his criminology work, forensic science has proliferated in law enforcement. Popular culture depictions have even created a phenomenon called the “CSI effect,” where jurors who watch police and crime scene procedural shows tend to put a high degree of expectation for and trust in forensics. But the validity of forensic science—particularly analysis of pattern and impression evidence, including handwriting, ballistics, blood spatter, and shoe and tire treads—has been called into question, with DNA testing overturning wrongful convictions built on such evidence.

Modern crime labs are primarily run by law enforcement agencies, with foundations in prosecution rather than fundamental science. Critics claim some techniques lack rigorous empirical testing; some, while scientifically sound, have results that can be misinterpreted by unqualified experts; and some are purely junk science. The use of bite mark analysis has embroiled the forensic science community in controversy, resulting in verbal sparring—and at least one assault—at national meetings. In 2015 an FBI audit revealed that its own testimonies on microscopic hair analysis contained errors in at least 90 percent of the cases reviewed.

Criticisms of modern forensics were precisely what Souder was trying to prevent. He maintained that without proper metrics and consistently trained interpreters, forensic science is merely a matter of opinion. An article he wrote for Army and Navy Journal in 1932 on firearms identification outlines what he deemed the minimum standards for his field. He notes that some critics of ballistics “condemn experts as absolutely unreliable and a menace to the state,” and admits that, given the inconsistency between forensics labs, such skepticism may be justified.

Forensic scientists, he writes, must be trained in metrology using advanced equipment, and expert witnesses must be evaluated on their ability to collect and interpret data. He also acknowledges that firearm identification methodology isn’t perfect: “It is the duty of the expert to admit his inability to solve the problem.”

Today there is no federal regulatory agency overseeing crime labs, and forensic science has drifted away from metrology. The National Institute of Standards and Technology is no longer involved with court cases, but its forensics division is dedicated to strengthening the field through research and improved standardization by focusing on science, policy, and practice.

“We have the legal community wanting finality. They want guilty, not guilty,” said Butler, now special assistant to the institute’s director of forensic science, during a symposium about Souder. “The scientific community, however, operates without that certainty.” So how do you explain that to someone who wants definitive answers? By “having standards and standardizing the way things are done,” as Souder advocated a century ago.

In 1954, after he had been fighting crime a micrometer at a time for four decades, one newspaper announced, “Here’s a bit of good news for the underworld”: Wilmer Souder, “one of the world’s foremost criminologists,” was retiring. He planned to spend his leisure time reading detective stories.

Updated 01/03/2023 to correct the name of Souder’s undergraduate school to Indiana University.