美国F1签证check的定义和check的目的

check这个词是广大签友们取的名字，在美国使馆的官方说法是行政审理，即就是administrative processing，主要是指对一些签证申请者的材料，进行更近一步的审查，其所花费的时间比一次性通过签证审查的人所需要的时间要多。主要审查的目的，根据美国国防院的说法，主要有四点：

1. 从根本上防止大杀伤性武器和导弹技术的扩散；
2. 限制在世界一些地区发展影响区域安全的传统武器;
3. 防止向恐怖主义国家输送军火和军民两用物品；
4. 确保美国在某些军事科技上的优势

以下为节选自原文，是参考了05年的一个帖子，仅作参考：

Media Note
Office of the Spokesman
Washington, DC
February 11, 2005

Extension of Validity for Science Related Interagency Visa Clearance
Visa applications for persons to study or work in certain sensitive scientific and technical fields are subject to an interagency clearance in Washington, DC, called Visas Mantis. Since 1998, the Visas Mantis clearance process has been used to screen against the illegal transfer of technology. Once the clearance process is complete and a visa is issued, the individual may apply for admission at a U.S. port of entry. Visas Mantis clearance and visa validity are different than the period of admission determined by a Department of Homeland Security officer at the port of entry. The U.S. Department of State, in consultation with the U.S. Department of Homeland Security, has extended the validity of Visas Mantis clearances for the F (student), J (exchange visitors), H (temporary workers), L (intracompany transferees) and B (tourist and business) categories of visas. This means that if the original visa has expired and a new visa application is filed to return to the previous study or work program in the United States, another Visas Mantis clearance may not be required. Consular officers have the discretion, if warranted, to request a Visas Mantis clearance during any visa adjudication. International students (F visas) who have received a Visas Mantis clearance and been issued a visa will benefit from having that clearance be valid for up to the length of the approved academic program, to a maximum of four years. If a student changes academic programs, the clearance will no longer be valid and a Visas Mantis review would be required should the applicant reapply for a new visa.