A truck driver barreled into a crowded Christmas market in Berlin on Monday night, killing at least 12 people and injuring scores more. The police believed it to be an attack, but the identity of any attackers and their motives were not immediately clear.

It was the most recent deadly episode to shake Europe in the last two years. Many recent attacks have been linked to jihadists, and some to the Islamic State, also known as ISIS. (The majority of terror attacks have been outside of Europe, primarily in Iraq and Syria.)