It's hard for someone outside the factory to get near the production line, so it's likely the crime took place from within the factory.
この場合、次のような言い方ができますよ。
ーIt's hard for someone outside the factory to get near the production line, so it's likely the crime was committed by someone from within the factory.
「工場外部の人が生産ラインに近づくのは難しいので、工場内部の人の犯行に違いない」
someone outside the factory「工場外部の人」
production line「生産ライン」
it's likely the crime was committed by someone from within the factory.「おそらく犯行は工場内部の人によって犯された」
ご参考まで！
It’s hard for outsiders to access the production line, so it’s likely an inside job.
It’s hard for outsiders to access the production line, so it’s likely an inside job.
Given the tight security, it must have been done by an insider.
・It’s likely an inside job.
（それは内部犯行である可能性が高いです。）
inside job は、従業員や関係者など「内部の人間による犯行」を指す非常に一般的な決まり文句です。
カナダ 
アメリカ合衆国 
