How did Shi Erqun who robbed the bank find the clue?

In fact, we found the suspect step by step according to the traces left by the suspect at the scene.

The person who can swing such a big hammer with the first hammer must be a person who has been engaged in manual labor for a long time (this physical evidence infers that he is a migrant worker)

The second scene was scratched by glass, leaving blood, hair and other remains. These evidences didn't have the technical conditions to detect useful information in the 1990s, but they could know the blood type at most, which didn't play much role in solving the case. But it's different now. I believe everyone will have an experience when they go to the hospital. For example, whether there are infectious diseases, hepatitis, HIV and so on. In the 1990s, there were not many migrant workers in Zhengzhou, most of whom were migrant workers in Henan Province. Therefore, modern detection technology uses the blood left by the scene to lock the suspects in some specific areas of Henan Province, so as to understand the situation in Henan (there were cases of selling blood to transmit diseases in some places before). Now more people are infected with HIV, but in Henan in the 1990s, this.

The third time was the robbery of more than 2 million in the 1990s. At that time, the money was huge, for ordinary people. Therefore, in an area of continuous carpet visit and investigation, clues will be found.