When the incident first happened, many did not know the truth.
"その事件が起きた当初は、多くの人が真実を知らなかった"
- When the incident first happened, many did not know the truth.
ある残酷な事件が起きました。その事件が起きた当初は、事件の全貌が明らかではなかったので、多くの人が真実を知りませんでした。
- There was a certain cruel case that happened. At the time it first happened, it wasn't clear what the full story was so many did not know the truth.
When that incident first took place, most people didn't know the truth that surrounding that story.
When that crime occurred initially, a lot people didn't know all the facts about it.
この場合、次のような言い方ができると思います。
ーWhen that incident first took place, most people didn't know the truth that surrounded that story.
「その事件が起きた当初、ほとんどの人は（事件に）まつわる話の真実を知らなかった」
When that incident first took place, ... で「その事件が起きた当初」と言えます。
ーWhen that crime occurred initially, a lot people didn't know all the facts about it.
「その犯行が起こった当初、多くの人は事実を知らなかった」
When that crime occurred initially, ...で「その犯行が起きた当初」とも言えます。
ご参考まで！
アメリカ合衆国 
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