A criminal disguised as a police officer attacked a cash truck carrying money from a company money and stole 300 million yen.

In Japan, there is a law sayingthat states that if a criminal is not found for 10 years after a crime is committed, the criminal's crime is considered to have never been achieved (?).

I don't understand what are you trying to say with the word "achieved".

Despite a large amount of evidence, we still do not know how the money was used (better: spent) or who the culprit was.

A total of 900 million yen was spentexhausted to find the culprit.

About tThe 300 Million Yen CaseRobbery

In English, I believe the case is referred to as a 'robbery'
https://en.wikipedia.org/wiki/300_million_yen_robbery

A criminal, disguised as a police officer, attacked a cash truck carrying company money and stole 300 million yen.

In Japan, there is a law sayingthat states that if a criminal is not found for 10 years after a crime ishas been committed, the criminal'sir crime is considered to have never been achieveddone.

The case has been featured in movies and other works of fiction because the culprit was never found despite the daring nature of the crime.

About tThe 300 Million Yen Case

In Japan, there is a law saying that if a criminal is not found forwithin 10 years afterof a crime isbeing committed, the criminal's crime is considered to have never been achieved is relieved of civil liabilities which means that s/he is able to openly admit guilt without fear of arrest or prosecution. .

面白そうです。

In Japan, there is a law saying that if a criminal is not found for 10 years after a crime is committed, the criminal's crime is considered to have never been achievcommitted.

When speaking about a crime, we don't really say " He achieved a murder" because it's not an accomplishment. Instead, we say he "committed a murder" because it's a crime.

In Japan, there is a law saying thatthat states if a criminal is not found for 10 years after a crime is committed, then the criminal's crime is considered to have never been achievoccurred.

- "saying that" is grammatically correct, but "that states" is more commonly used.
- achieveという動詞は、通常に努力や勇気、能力によって「達成」・「目的をはたす」の意味です。ポジティブな意味合いを持っています。なので、"occurred"か"happened"は十分の言葉だと思います。