Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: In 1915 Berlin, the German high command is worried about ally Turkey. Recent British attacks on the vital Dardanelles shows signs of inside knowledge. Von Sturm, the head of German intelligence, is ordered to find out if Ali Bey, the Turkish commander of the region, is the traitor.
As his best agent has not been heard from in several weeks, von Sturm assigns Kruger the task. Shortly afterwards, Annemarie, known by the code name "Fräulein Doktor", returns after completing her mission. She also informs von Sturm that fellow spy Mata Hari has fallen in love and therefore can no longer be trusted. She recommends that an incriminating message be sent using a code that she knows has been broken by the Allies. In addition, she uncovers Kruger as a British double agent known as K-6.
When Kruger is arrested at his dentist contact's office, another patient, American medical student Douglas Beall, is also taken into custody, though he is later released. Just to be sure though, von Sturm orders Annemarie to make sure Beall is innocent. She arranges to be rescued from an unwanted "suitor" by Beall, who invites her to his hotel suite. During the course of the evening, he confesses he has fallen in love with her, now going by the name Helena Bohlen. Helena is attracted to him, but when she reads a coded message from von Sturm informing her that he has taken her advice regarding Mata Hari, she abruptly leaves.
Beall persists however. When Helena boards the train to Constantinople, he follows her on the spur of the moment and continues courting her, despite her half-hearted attempts to discourage him. Her assistant Karl watches with growing concern. As they near the Turkish border, she orders Karl to return to Germany so Beall can use his visa.
What is the name of the person whose best agent has not been heard from in weeks?