熟人 (shú rén): Acquaintance (Noun) — Meaning & Stroke Order

Based on these clues, we can speculate that the murderer might be someone he knows.

How do you define the difference between a friend and an ordinary acquaintance?

Why did you suddenly turn your head? Did you see someone you know?

In our daily chats, the broad sense of 'friend' includes acquaintances and colleagues.