Most of the cases Souder worked on came from federal agencies. But there was at least one exception. On May 9, 1932, a Lieutenant R. A. Snook of the New Jersey State Police met with Souder at the bureau’s labs in Washington, D.C. Snook was the chief investigator in the Lindbergh case, and he brought with him the original ransom notes for Souder to photograph and analyze.

A few days later, Snook filed a report of this meeting with his police agency. “Dr. Souder explained that arrangements had been made for the work to be kept strictly confidential,” the report said. “The case was to be known to the Bureau of Standards as the Adamson case,” a fictitious name that Souder invented to throw busybodies off the trail.

The report went on to say that when Souder introduced Snook to people at the bureau, he introduced him as, “Mr. Martin, from New York.”

The Chicago Daily Tribune reported Souder’s involvement later that month, so in the Lindbergh case, at least, the information didn’t take long to leak out. But Souder’s notebooks are full of obscure references.

“Who knows what other famous cases he worked on,” Butler said.

On September 6, 1932, Souder must have been working on the Lindbergh case. His notebook entry for that day says, “see Adamson Volume.” Butler and Frederick-Frost searched everywhere they could think of for the Adamson volume. “I was obsessed with finding it,” Frederick-Frost said.

So far, at least, they haven’t.

The National Archives

Their next big break came later that year, when Frederick-Frost was searching through binders full of old archive transfer records at NIST. In one of them, she found a half-century-old transfer slip that documented a shipment to the National Archives containing more than 10,000 glass plate negatives. Other shipments contained detailed reports of laboratory test results. At least some of these photographs and reports, she was sure, would be from Souder’s lab.

She and Butler visited the National Archives in College Park, Maryland, several times in late 2015. Unfortunately, they learned that detailed reports related to identification of handwriting, typewriting and bullets—these were Souder’s areas of expertise—had been discarded in April 1944, presumably to make room in the collection for new material. But the glass plate negatives were there, and an archivist named William Wade gamely assisted by hoisting and carting heavy boxes full of those plates to the viewing area.

“We were at the end of a long day and I said, ‘Billy I have just one more pull, I’m sorry,’” Frederick-Frost said. He came back with a box that looked different from the others. When new material arrives at the facility, archivists repackage it in preservation-quality boxes. This was an old cardboard box that looked as if it had not been opened for decades.

Butler and Frederick-Frost, wearing white cotton art-handling gloves, opened the box. It was stuffed with shredded newsprint. Pushing that aside, they dug out two smaller boxes sealed with black tape. Scribbled across the top of those, in the same penciled handwriting that fills Souder’s nine notebooks, they saw the words, “Baby Lindbergh Kidnapping Case.”

“We were ecstatic,” Frederick-Frost said.