On page 243 of its 646-page main report (available in Japanese only, for now), the National Diet Independent Investigation Commission on the Fukushima I Nuclear Power Plant accident speculates on the causes of the spikes in radiation levels at monitoring stations in March 2011. (This particular page was mentioned in a tweet by Professor Ryugo Hayano at Tokyo University Physics Department.)
The Commission seems to have concluded that Reactor 1 released large amounts of radiation on March 15 and 16, and again on March 21, 2011. It also suspects the fuel debris (corium) re-melt of Reactor 3 for the spike of radiation on March 21 and 22, 2011. In the latter, the Commission cites a research paper by Fumiya Tanabe published earlier this year but was reported in August last year.
(Tanabe is the one who also said Reactor 2 Suppression Chamber and Dry Well were damaged in the earthquake, and the efforts to vent released a large amount of radioactive materials from the breaches.)
The particular section discusses the recriticality issues and the hydrogen explosion in Reactor 4. The Commission writes in the subsection titled "a. outside monitoring data" that says (my personal translation):
We referred to the outside monitoring data regarding the short-lived nuclides that are produced should the recriticality occur.
At the CTBT Monitoring Post in Takasaki City in Gunma Prefecture, increased amounts of nuclides, including the short-lived nuclides, were sampled and observed around March 15 . However, we cannot deny the possibility that they were produced during the course of the regular reactor operation [as opposed to being produced by recriticality] or that they were decay products of such nuclides.
According to the measurement by the Japan Chemical Analysis Center, the amount of tellurium-129, iodine-132, and xenon-133 showed rapid increase, but it is likely that they were originated in the normal reactor operation.
From these monitoring data, we cannot find the data that clearly shows recriticality was taking place. However, it is apparent that there were large-scale releases of radioactive materials from Reactor 1 on March 15 and 16, and on March 21. The release of radioactive materials between March 15 and 17 is likely to have been caused by the damaged S/C (Suppression Chamber) and D/W (Dry Well) of Reactor 2, and by the Reactor 3 vent and the hydrogen explosion. As to the rise on March 21 and 22, we need to consider the possibility of the fuel debris re-melt in Reactor 3.
Well, in other words, we still don't know what happened and how it happened after 16 months. The Diet Commission quotes the data and research papers, but all it can do is to suspect everything, and ends up listing all possible causes for the releases of radioactive materials. I read the tweets between the researchers (including Professor Hayano who tweeted this particular page). They are nowhere near any conclusion.
The only thing that is notable for me is the mention of Reactor 1 as a large source of radioactive materials. Both TEPCO's and the government institutions' analyses indicate the release from Reactor 2 was an order of magnitude bigger than other reactors. I guess I will have to read the 646-page report to see if they have the data to back up their assertion about Reactor 1.