Intel warned

Attack Sets Off Hunt for Tunisian Who Had Slipped Germany’s Grasp

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.