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By the early 1940s, US Army Intelligence was already aware of allegations that Tsien was a Communist, but his security clearance was not suspended. However, on 6 June 1950, his security clearance was revoked and Tsien was questioned by the FBI. Two weeks later Tsien announced that he would be resigning from Caltech and returning to China, which by then was effectively governed by the Communist Party of China led by Mao Zedong.

By the early 1940s, US Army Intelligence was already aware of allegations that Tsien was a Communist, but his security clearance was not suspended. However, on 6 June 1950, his security clearance was revoked and Tsien was questioned by the FBI. Two weeks later Tsien announced that he would be resigning from Caltech and returning to China, which by then was effectively governed by the Communist Party of China led by Mao Zedong.

In August, Tsien had a conversation on the subject with the then Under Secretary of the Navy Dan A. Kimball, whom Tsien knew on a personal basis. After Tsien told him of the allegations, Kimball responded, "Hell, I don't think you're a Communist", at which point Tsien indicated that he still intended to leave the country, saying "I'm Chinese. I don't want to build weapons to kill my countrymen. It's that simple." Kimball then said, "I won't let you out of the country."

In August, Tsien had a conversation on the subject with the then Under Secretary of the Navy Dan A. Kimball, whom Tsien knew on a personal basis. After Tsien told him of the allegations, Kimball responded, "Hell, I don't think you're a Communist", at which point Tsien indicated that he still intended to leave the country, saying "I'm Chinese. I don't want to build weapons to kill my countrymen. It's that simple." Kimball then said, "I won't let you out of the country."

8月，钱学森与当时的海军副部长丹·A·金博尔(Dan A. Kimball)就这个问题进行了交谈，钱学森私下认识金博尔。钱学森告诉他这些指控后，金博尔回应说，“见鬼，我不认为你是共产主义者”。钱学森暗示，他仍然打算离开中国，并说“我是中国人。”，我不想制造杀死我同胞的武器，就这么简单。”金博尔接着说，“我不会让你回中国的。”

同年8月，钱学森与当时的海军副部长丹·A·金博尔(Dan A. Kimball)就这个问题进行了交谈，钱学森与金博尔有私交。钱学森告诉他这些指控后，金博尔回应说，“见鬼，我不认为你是共产主义者”。钱学森暗示，他仍然打算离开美国，并说“我是中国人，我不想制造杀死我同胞的武器，就这么简单。”金博尔接着说，“但我不会让你回中国的。”

After the firm in charge of arranging Tsien's move back to China tipped off U.S. Customs that some of the papers encountered among his possessions were marked "Secret" or "Confidential," U.S. officials seized them from a Pasadena warehouse. The U.S. Immigration and Naturalization Service issued a warrant for Tsien's arrest on 25 August. Tsien claimed that the security-stamped documents were mostly written by himself and had outdated classifications, adding that, "There were some drawings and logarithm tables, etc., which someone might have mistaken for codes." Included in the material was a scrapbook with news clippings about the trials of those charged with atomic espionage, such as Klaus Fuchs. Subsequent examination of the documents showed they contained no classified material.  Weinbaum's trial commenced on 30 August and both Frank Oppenheimer and Parsons testified against him. Weinbaum was convicted of perjury and sentenced to four years. Tsien was taken into custody on 6 September 1950 for questioning

After the firm in charge of arranging Tsien's move back to China tipped off U.S. Customs that some of the papers encountered among his possessions were marked "Secret" or "Confidential," U.S. officials seized them from a Pasadena warehouse. The U.S. Immigration and Naturalization Service issued a warrant for Tsien's arrest on 25 August. Tsien claimed that the security-stamped documents were mostly written by himself and had outdated classifications, adding that, "There were some drawings and logarithm tables, etc., which someone might have mistaken for codes." Included in the material was a scrapbook with news clippings about the trials of those charged with atomic espionage, such as Klaus Fuchs. Subsequent examination of the documents showed they contained no classified material.  Weinbaum's trial commenced on 30 August and both Frank Oppenheimer and Parsons testified against him. Weinbaum was convicted of perjury and sentenced to four years. Tsien was taken into custody on 6 September 1950 for questioning

While at Caltech, Tsien had secretly attended meetings with J. Robert Oppenheimer's brother [[Frank Oppenheimer]], [[John Whiteside Parsons|Jack Parsons]], and [[Frank Malina]] that were organized by the Russian-born Jewish chemist Sidney Weinbaum and called Professional Unit 122 of the Pasadena Communist Party.<ref>[[Ray Monk]], ''Robert Oppenheimer: A Life Inside the Center'' [[Random House]] {{ISBN|978-0-385-50407-2}} (2012)</ref>  Weinbaum's trial commenced on 30 August and both Frank Oppenheimer and Parsons testified against him.<ref>[[George Pendle]], ''Strange Angel: The Otherworldly Life of Rocket Scientist John Whiteside Parsons'' [[Mariner Books]] (2006) {{ISBN|0-297-84853-4}} p.&nbsp;291.</ref> Weinbaum was convicted of perjury and sentenced to four years.<ref>Chang (1995), p.&nbsp;159.</ref> Tsien was taken into custody on 6 September 1950 for questioning<ref name="MJ550913" /> and for two weeks detained at [[Federal Correctional Institution, Terminal Island|Terminal Island]], a low-security United States federal prison near the ports of Los Angeles and [[Long Beach, California|Long Beach]].

While at Caltech, Tsien had secretly attended meetings with J. Robert Oppenheimer's brother [[Frank Oppenheimer]], [[John Whiteside Parsons|Jack Parsons]], and [[Frank Malina]] that were organized by the Russian-born Jewish chemist Sidney Weinbaum and called Professional Unit 122 of the Pasadena Communist Party.<ref>[[Ray Monk]], ''Robert Oppenheimer: A Life Inside the Center'' [[Random House]] {{ISBN|978-0-385-50407-2}} (2012)</ref>  Weinbaum's trial commenced on 30 August and both Frank Oppenheimer and Parsons testified against him.<ref>[[George Pendle]], ''Strange Angel: The Otherworldly Life of Rocket Scientist John Whiteside Parsons'' [[Mariner Books]] (2006) {{ISBN|0-297-84853-4}} p.&nbsp;291.</ref> Weinbaum was convicted of perjury and sentenced to four years.<ref>Chang (1995), p.&nbsp;159.</ref> Tsien was taken into custody on 6 September 1950 for questioning<ref name="MJ550913" /> and for two weeks detained at [[Federal Correctional Institution, Terminal Island|Terminal Island]], a low-security United States federal prison near the ports of Los Angeles and [[Long Beach, California|Long Beach]].

On 26 April 1951, Tsien was declared subject to deportation and forbidden from leaving Los Angeles County without permission, effectively placing him under house arrest.

On 26 April 1951, Tsien was declared subject to deportation and forbidden from leaving Los Angeles County without permission, effectively placing him under house arrest.