1942年2月20日,美国反间谍人员检查一封寄往葡萄牙的邮件时,发现这是德国间谍传送的秘密情报。几小时后,邮件被送到华盛顿联邦调查局实验室。这是一个男人写给朋友的信,写在一张打字纸上。一个研究密写墨水的专家,用一块浸透化学药剂的海绵,查出写在打字纸空白处的情报,内容是纽约港内 On February 20, 1942, when U.S. anti-spy officers checked a mail addressed to Portugal, they found it a secret message sent by German spies. A few hours later, the mail was sent to the FBI lab. This is a man’s letter to a friend, written on a typing paper. An expert with a close-up of inks, found a sponge that was soaked in chemicals that information was found in the blank space on typed paper, in New York Harbor