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!