Lastbil har kört in i julmarknad i Berlin 2016-12-19, 12 döda

Fyra personer gripna i Tyskland efter attentatet

Vid 09-tiden på torsdagsmorgonen har fyra män har gripits efter att federala poliser har stormat en lägenhet vid 06-tiden i staden Dortmund.

The identity of the Tunisian, Anis Amri, immediately alarmed intelligence officials from Europe to Washington. German officials acknowledged that Mr. Amri was known to have links to a radical Salafist preacher and had been in their custody pending deportation proceedings after being caught with fake papers. He was freed, even though he was considered potentially dangerous by the authorities.

He also appeared on the radar of United States agencies, according to American officials. He had done online research on how to make explosive devices and had communicated with the Islamic State at least once, via Telegram Messenger, said the officials, who spoke on the condition of anonymity because of the investigation. He was also on a United States no-fly list, the officials said on Wednesday evening.

The Tunisian was considered by security officials to be a gefährder, someone deemed likely to endanger the state.

There are 549 people listed as dangerous Islamists, not all living in Germany, an Interior Ministry official said on the condition of anonymity, which is habitual here when discussing such matters.

German intelligence and law enforcement authorities, like those in France and Belgium, say they are overwhelmed by what it would take to keep constant tabs on such large numbers of people. They are under various degrees of surveillance, but a full-time watch on all of them would require up to 40 officers per individual — impractical, security sources say.