Hitler's secret plot to blow up trains, factories, and bridges in the Northeast was dubbed Operation Pastorius, after the founder of Germantown.
The local reference was appropriate.
When the two four-man teams of Nazi saboteurs traveled by submarine to America in 1942, they carried the name of their U.S. contact written on a handkerchief - a man quite familiar with Philadelphia.
"Pas. Krepper," as he was called, was to provide money and lodging while they prepared to attack sites including a cryolite metals plant in Philadelphia, the Horseshoe Curve railroad pass near Altoona, and Pennsylvania Station in Newark, N.J.
"Pas" stood for pastor.
Krepper was the Rev. Carl Emil Ludwig Krepper, who had emigrated to America, become a citizen, and served congregations in the city and South Jersey before conspiring in a wartime terror plot.
For decades, details about Krepper's life and his role in the failed scheme remained largely unknown. But 12 years ago, the Rev. J. Francis Watson, president of the Lutheran Archives Center in Philadelphia, discovered a sealed envelope at the archives, at the Lutheran Theological Seminary in Mount Airy.
"There was one single press clipping on the arrest," said Watson, 52, of Manchester, N.J. "I thought, 'This is strange. I never heard of this guy.' "
With that, Watson embarked on a mission to uncover the truth about Krepper, a fellow Lutheran.
He scoured newspaper articles and church records. Then, in 2012, he hit the jackpot: The FBI granted his request and sent him thousands of pages of newly declassified files on the pastor and the plot.
Watson chronicles the minister's life, downfall, and arrest 70 years ago in a new book, The Nazi Spy Pastor: Carl Krepper and the War in America, just published by Praeger.
"He allowed the Nazi ideology to pervert his sense of calling as a pastor," Watson said in an interview this month. "God and the Reich were closely identified in his mind."
A short, slight man who was rarely without his brown briefcase, the German-born Krepper elected to move to the United States in 1909, answering a call for German-speaking ministers.
Krepper sailed on the SS President Grant in search of a new life, and settled in Philadelphia, the home base of the program seeking clergy.
He studied at the Lutheran Theological Seminary for three months in 1909 and was ordained in 1910. He was called to serve at St. John's Lutheran Church in Williamstown, N.J., and remained there for about a year before becoming pastor at Friedens (German for "peace") Lutheran Church in Philadelphia.
Krepper's tenure at the Kensington congregation was fairly impressive, Watson said.
The congregation grew to include 200 families, expanded its physical plant, and started a men's baseball league. Krepper soon became statistician for the German Conference of the Ministerium of Pennsylvania.
He preached only in German for the 13 years he remained at the church, said John Peterson, curator of the Lutheran Archives Center of Philadelphia.
Krepper is mentioned in the church history in connection with the congregation's reducing its substantial debt during his tenure, said the Rev. Raymond Kvande, former pastor at Peace Lutheran Church, which closed in 1997.
Krepper, who became an American citizen in 1922, lived nearby on Allegheny Avenue with his wife, Marie, and son, Ernst.
But scandal scuttled Krepper's tenure at Friedens Lutheran Church. He and his wife divorced and he married the ex-wife of the man who had sponsored his emigration to the U.S. He resigned from the church and moved on to serve the rest of his career in central New Jersey, where news of Hitler's rise began to stir Krepper's long-dormant feelings of German nationalism.
Krepper took trips to Germany and became involved in anti-Semitic groups.
Most German Americans were loyal to their adopted country, said Paul Rorem, a professor of ecclesiastical church history at Princeton Theological Seminary. They were eager to become Americans. But some pastors in Germany became sympathetic to the Nazi cause, Rorem said. Eventually, Krepper became one of them.
Krepper was serving as pastor of Newark's First German St. John's Evangelical Lutheran Church in 1935 when he left the congregation to move back to Germany.
There, he became embroiled in Operation Pastorius - named for Francis Daniel Pastorius, the German-born Quaker lawyer and poet who founded the settlement of Germantown.
Krepper sailed back to the U.S. in 1941, this time as the man who would set up safe houses for the saboteurs.
The safe houses Krepper organized were Newark properties owned by one of Krepper's three American girlfriends. (He had left his wife behind in Germany.)
In 1942, the FBI uncovered the plot after a series of errors by the German agents, and two of the saboteurs eventually decided to turn themselves in. Krepper was followed for two years as authorities collected evidence, then nabbed him in a sting.
He told an undercover agent: "My heart is still in Germany."
Charged with, among other things, conspiracy to commit sabotage and trading with the enemy, Krepper was convicted in March 1945. He served six years in a federal prison in Lewisburg, Pa.
Two of the saboteurs were deported. Six were executed.
Krepper was released in 1951 and moved to the Berkshires, where one of his girlfriends owned property. He died in 1972 and is buried in a pauper's grave in Massachusetts.
Several years ago, Watson presented his research before a women's group in central New Jersey.
Helen Gehring, 92, was there. She said she remembered Krepper as the "stern" man who was pastor of her church when she was a youngster.
Watson called his research into that stern pastor's background a historical mystery that was intriguing to solve.
"We learn from our past - the good and the bad," Watson said. "And this is a classic case of that to me."