In early February 1940, Mormon FBI agent James Ellsworth left his home in Huntington Park, California, prepared for a normal day of work. Instead, he was met by a man with a message from John Edgar Hoover, director of the FBI in Washington, D.C., instructing him to take the next possible plane to New York City.

“I asked Dick what the case was and he said Mr. Hoover would not say,” Ellsworth’s personal diary read. “I asked him how long I should prepare to be gone and he said he had no idea.”

Ellsworth immediately returned home, packed a suitcase for a two-week trip, picked up his plane ticket and went to New York.

“When I arrived at the airport at New York City, I was met by two agents whom I knew,” Ellsworth's diary continued. “They put me into an automobile and drove me into New York City. On the way they took away my gun, my identification badge and credentials and all identifications cards. They gave me a new set of identification cards and a fictitious name and gave me a little background of the case I was to work on.”

It was that day that Ellsworth met German-born William G. Sebold, the first double agent in FBI history, as Ellsworth took part in the war against Nazi Germany. Ellsworth, who had served a mission for The Church of Jesus Christ of Latter-day Saints in Germany, including serving in the city where Sebold was born, would spend 16 months with Sebold as his handler on a case that would eventually lead to the arrest of 33 spies in what is still the largest espionage case in American history.

This was the way James Ellsworth’s son, Tom, remembers his father beginning his telling of the Sebold case — a story about a case and a double agent that are not much remembered today, now told in Peter Duffy’s latest book, “Double Agent: The First Hero of World War II And How The FBI Outwitted And Destroyed A Nazi Spy Ring" (Scribner, $28).

“We knew his experience, but we didn’t have the background,” Tom Ellsworth said in a phone interview. “The whole part of the way the nation felt about war, the situation with the FBI at the time. … So we had his view of it, but we didn’t have the complete picture, which Peter Duffy so nicely laid out.”

While Duffy lays out the thrilling and complex history of the events happening in the United States and Germany in the 1940s, it was James Ellsworth's journals that gave him a clear timeline of events within the Sebold case.

“It was such a kind of resource you can’t imagine that you could find,” Duffy said of the journals in a phone interview. “It was extremely helpful in placing where the case was at certain points, where Sebold was living, what the order of events was.”

The case was extremely hard to write a coherent narrative of, Duffy said, because it was so long and complex. Duffy said he had to learn everything about the 33 who were convicted in order to really get down to the important things.

“Jim Ellsworth was loyal and honorable and dedicated to the case, and we can see that in looking at the fact that he didn’t begin writing the diary until the arrests had been made," Duffy said. "To write it during the case … it could have gotten into the wrong hands or jepordized the case in various ways. And then he returned to his diary. It was probably the longest he had not kept a diary in his life. Ellsworth was a unique figure.”

For the Ellsworth family, the journals and letters between their parents surrounding the case, kept for family history, were something they thought about but never got around to publishing — part of the reason being the amount of research and information needed to do the story justice.

“When Peter called, I knew he would tell it correctly,” Tom Ellsworth said. “It might not be the way we might tell it, it might not have the spiritual aspects that we might have felt, but nevertheless, it would be told accurately. … I trusted him with the material.”

While the spiritual aspects of the story may not have had a place in Duffy’s book, they were defining for the Ellsworths.

“Dad was in the FBI for 20 years. He was in some very scary situations where he was in danger and never had to draw his gun,” Tom Ellsworth said. “He always felt like he was protected. Now, my mother didn’t feel that way, but those were the times when she taught me how to pray.”

Tom Ellsworth said that there were many times when his father didn’t come home when he was supposed to or when his mother would call him at the office and discover he was out on assignment.

“She had incredible faith,” Tom said of his mother. “She would tell me to go to my closet and pray for father’s safety. And so I would. And of course, he always came home fine.”

Although the spiritual experiences didn't fit into the book, Duffy includes James Ellsworth's dedication to his faith many times.

“(Ellsworth) kept a diary all his life. His diaries are an important historical artifact, for particularly the Mormon experience,” Duffy said of Ellsworth, who would later be called by President Harold B. Lee as a mission president in Germany. “He was deeply connected to his Mormon faith and wrote about it in his diaries, which encompasses a large part of the 20th century.”

"Double Agent" gives readers a captivating glimpse into a nearly forgotten yet important piece of history.

“I think that the point of the story is that it happened in 1941, which is a period when the war was going on in Europe but the United States was not involved yet," Duffy said. "This case shows that it was one double agent and a crew of FBI agents who were the few people who were actually fighting World War II. The first Americans engaged in World War II were these individuals.”

Hikari Loftus is a graduate of the University of Utah. She blogs at FoldedPagesDistillery.blogspot.com.