NIE-31 begins: 

"The intelligence organizations of the Departments of State, the Army, the Navy, the Air Force, the Joint Staff, the Federal Bureau of Investigation, the Atomic Energy Commission, and representatives of the Coast Guard, the Department of Agriculture, the Bureau of Customs, and the Public Health Service participated in this estimate. All members of the Intelligence Advisory Committee concurred in this estimate on 30 August. 

THE PROBLEM 

To estimate for the period mid-1951 to mid-1952 the vulnerability of the US to Soviet clandestine attack with weapons of mass destruction prior to or concurrent with the outbreak of hostilities on the basis of Soviet capabilities or such attack and US capabilities for detecting and preventing an attack. 

CONCLUSIONS 

1. The Soviets have substantial capabilities for the employment of atomic, chemical, and biological weapons for clandestine attack upon the continental US. 

2. The US is vulnerable to such clandestine attack because existing and presently planned security measures do not provide adequate assurance that certain methods of clandestine attack would be detected and prevented. 

3. In a clandestine attack on the US the USSR would probably attempt simultaneous delivery of a number of atomic weapons, possibly by several methods. 

a. The most likely method of attack, because the most feasible and potentially most effective, would be the used of disguised TU-4 aircraft [Soviet copies of US B-29 bombers almost identical in appearance to the US model - JC] to deliver atomic weapons to a number of targets simultaneously as the initial act of general hostilities. 

b. The delivery of atomic weapons into key harbors by merchant ships is feasible and therefore constitutes a serious threat. 

[...] 

d. The launching of guided missiles with atomic warheads from merchant ships or submarines against near-coastal targets is a possibility..... 

DISCUSSION 

Delivery By Disguised Aircraft 

16. Because of its resemblance to the US B-29, the Soviet TU-4 could be disguised with US markings and employed for clandestine delivery of atomic bombs. Flying a one-way mission, the TU-4 has sufficient range to reach every important target in the US and the USSR has an adequate number of TU-4s and trained crews to perform such missions. 

17. Present flight regulations of the Civil Aeronautics Administration and the military services require that both military and civilian aircraft follow a previously filed flight plan and enter the US by specified routes. Aircraft violating these requirements, if detected by our radar screen or other means, are intercepted in flight by USAF fighters. Our radar screen now covers Alaska and the northeastern portion of the US, and is being extended to cover significant gaps. 

18. A small number of disguised TU-4s, by taking advantage of the gaps in our radar screen, might escape detection. This would greatly increase the probability of a successful attack on high priority targets, such as the Washington area, for the purpose of paralyzing the top military and civil command a few hours prior to the initiation of hostilities elsewhere. 

19. The USSR also could undertake clandestine attack with civilian aircraft of a type used by US or foreign transoceanic airlines. Such aircraft would have a greater chance of escaping detection and identification inasmuch as civilian aircraft are not equipped with IFF [coded radio identification gear - JC]. However, employment of civilian aircraft is less probable since, at present, neither the the USSR nor any of the Satellites are known to possess suitable aircraft, and acquisition from either a US or foreign concern would increase the risk of compromising the operation." 

************************** 

Top Secret Special Estimate 10, "Soviet Capabilities For A Surprise Attack On The Continental United States Before July 1952" is even more intriguing, however. To quote: 

" The intelligence organizations of the Departments of State, the Army, the Navy, the Air Force, and the Joint Staff participated with the Central Intelligence Agency in producing the section of this estimate covering Direct Military Attack....The section on Clandestine Attack with Weapons of Mass Destruction...is based on NIE-31....The section on Subversive Operations, Sabotage and Civil Disturbances...was prepared by and has the approval of the Interdepartmental Intelligence Conference. The members of the Intelligence Advisory Committee concurred in this estimate on 10 September 1951. 

THE PROBLEM 

To estimate Soviet capabilities for weakening, disrupting or destroying the war potential of the United States by a surprise attack against the Continental United States before July 1952. 

ASSUMPTIONS 

For the purpose of this estimate it is assumed that: 

a. A Soviet surprise attack on the US would be designed to cause the maximum possible reduction in the capability of the US to wage offensive war. 

b. The scale and nature of the Soviet effort against the US would not be significantly affected by possible simultaneous Soviet military campaigns in other areas. 

c. The USSR would not avoid employing any weapon and tactic because of US capabilities for retaliation in kind.... 

AIR ATTACK 

1. Atomic bombardment with long-range aircraft is the most effective among the various types of potential surprise air operations against the US within Soviet capabilities during the period considered in this estimate. 

2. Other possible types of surprise air attack, such as conventional bombing with high explosives, guided missiles launched from Soviet-controlled territory and the employment of free balloons will not constitute a serious threat during the period of this estimate. Although chemical and biological weapons might be delivered by long-range aircraft, these weapons are better suited to clandestine or sabotage attack. It is considered unlikely that the USSR will possess a hydrogen bomb during the period of this estimate. 

 [...] 

SUBVERSIVE OPERATIONS, SABOTAGE, AND CIVIL DISTURBANCES 
[footnote: "The following estimate has been made by the Interdepartmental Intelligence Conference composed of the Directors of the Federal Bureau of Investigation; G-2, Army; Office of Naval intelligence; and Office of Special Investigation, Air Force."] 

Groups and Individuals Available for Soviet Subversive Purposes 

34. To aid in its attempts to disrupt and frustrate our defensive and counter-offensive efforts in the circumstances of a surprise attack, the USSR has a very formidable ally within our own camp: the Communist Party, USA. The members of that organization, now estimated at 37,000, by virtue of their total devotion to the principles of Marxism-Leninism and to the Soviet 'fatherland,' are committed to defend the USSR 'unswervingly,' with all means at their disposal and at any price. 

[...] 

Communist Organizational Tactics in the Present Period 

37. The Communist Party, USA, has always conducted some of its activities on an underground basis, but since 1947, that basis had been enlarged considerably through the taking of elaborate 'security measures' designed to provide additional cover for its activities. Now, since the indictment of the National Board members (17 June 1948)...the Party has gone even further underground. A Soviet surprise attack would surely complete the 'descent.' 

[...] 

Communist Action to be Anticipated in Event of Soviet Surprise Attack 

40. In the event of a surprise attack, the Communist Apparatus may be expected to make coordinated attempts immediately to destroy: 

a. our will to resist 
b. our means of resisting 

41. Psychological warfare directed toward the destruction of our will to resist (or toward causing fatal hesitation or confusion in repelling the attack and launching the counter-offensive) might take the form of widespread circulation (effected, e.g., through newspapers with 'forged' mastheads and titles, broadcasts over seized radio stations, etc.) of false reports and rumors concerning: the strengths and initial successes of the enemy; the destruction and/or capture of important cities; the slaughter of millions of military and civilian citizens by means of both powerful and insidious weapons; the surrender of important units of our Armed Forces; the existence of total confusion among our military and Government leaders; and the unmasking (forced or spontaneous) of Communists in high military and 
Government posts. Such efforts, if successful, might cause seriously disruptive civil disturbances, such as race riots, a revolutionary situation on the home front, and a lowering of military and civilian morale."