There is ironclad evidence that this incident was orchestrated to provide a pretext for invasion, including but not limited to Soviet and Russian admissions of the fact in the late 1980s and 1990s. The only thing that is somewhat in doubt is whether there were real casualties or even any shelling, or if the whole incident was simply invented wholesale by the Soviets.

First, Finnish leadership and troops had every reason to try to avoid war, and precisely zero reasons to attack the Soviets. So there was absolutely no credible motive whatsoever for such an attack.

Second, there is more than ample and well corroborated documentary and eyewitness evidence that 1) as ordered precisely for the reason to avoid even accidentally shelling Soviet territory and thus providing a casus belli, Finnish artillery units had been withdrawn away from the border and were not within range of Mainila; and 2) by plotting the directions from where the firing sounds were heard by various border observation posts, the location of the firing units could be definitively established to be within the Soviet side of the border.

In 1992, Russian historian Pavel Aptekar was able to study the war diaries of the Soviet 70th sharpshooter division, stationed close to Mainila at the time, but could not find any mention of any casualties or even of the attack. While he noted that the war diary may not have been the original one, his conclusion was that the casualties at least were purely a Soviet invention, and that the shelling may not have occurred either. Another historian, Vladimir Baryshnikov, has found archival reports that say that the incident was reported to the Leningrad military district from Moscow, rather than from the Leningrad military district to Moscow, which strongly suggests that the incident was orchestrated. However, eyewitness testimony, including from the Soviet side, suggests that 5-7 shots were fired and they landed at Mainila, about 800 meters from the Finnish border. (Helsingin Sanomat 6th Nov 1992). Finnish eyewitnesses, such as private Unto Sundvall, whose testimony can be found in the Finnish National Archives, did not observe any casualties. (Suomen Kuvalehti, 26th Nov 2015)