The largest problem around this time was an abduction by North Korea. There were several legal cases that North spy assets were prosecuted in the court until 1981, though it was halted afterward. At the same time, the abduction was diminished, but it was increased around 1985.
This might have been a subject of their deal between the Japanese police and North Korea, though there is no proof. In those days, there were many traces that North Korea came to the Japanese shore by boat to abduct Japanese for their future operation.
However, the abduction case was not officially permitted and mainly ignored by the Japanese mainstream as it was unconceivable. I have doubted the police intelligence hid some intel which was why this did not become a social issue when it really occurred.
The police should have known a true identity of missing cases, but there should have been reasons to hide it. Retrospectively, this is the biggest issue when they made a deal, therefore this might a center of the deal.
However, there was another issue related to the left extremist which was actually in between of them. The extremists have been connected to North Korea, which was apparent that some of the Japanese Red Army flew over with a hijacked airplane.
Some of the left extremist activity has been supported by North including an operation against the Japanese politics, some of which might have had disastrous outcomes, such as an assassination of the important figure. This possibility was not deniable, whose exemption triggered a North Korean concession to the police.
Some of those left extremists have been spies of the police intelligence, hence in this sense, they were basically connected in this path as well. Furthermore, the police intelligence required a sabotage of North operatives and left extremists to show their necessity to the government. Their relationship has been so interwound from many perspectives.
I did not think the Japanese police intelligence was controlled by North Korea, but they have given many freehand to the North operative to conduct an operation in Japan. As a result, some of the police intelligence operation have been influenced by the Northern intention.
There were several possibilities for this outcome, such as an informational control of North to the Japanese intelligence, or extremist spy conducting outsourced intelligence operation with Northern intention.
In any way, their relationship was so tight, which was even fortified by the shared interest of Pachinko.