Richard Rashke is the author of "Useful Enemies: John Demjanjuk and America's Open-Door Policy for Nazi War Criminals."

This month, the Associated Press exposed yet another alleged Nazi collaborator, Michael Karkoc, a carpenter who had been living quietly in Minnesota for decades. During World War II, the news service reported, he was "the top commander of a Nazi SS-led unit accused of burning villages filled with women and children."

Karkoc's son has vehemently denied his father had such Nazi connections, but even if it turns out Karkoc, now 94, was a collaborator, it would not be all that surprising that he managed to immigrate to the United States. Despite stated policies aimed at keeping Nazis and those who worked with them out of the country in the years after World War II, many slipped through.

Estimates of the number of former Nazi war criminals and their collaborators who entered the U.S. during the hectic postwar years range widely from 1,000 to 10,000. Based on my own research, I would put the number at somewhere between 3,000 and 5,000....