What constitutes a threat?

The manhunt for Anis Amri spread to the state of North Rhine-Westphalia, where he is considered an Islamist threat.

A person who meets the criteria of a so-called threat is only loosely defined, according to the German government's answer to a request for information: "A threat is a person who can be justifiably assumed in the case in question to carry out politically motivated crimes of considerable importance."

Or, as Martin Kahl from Hamburg University's Institute for Peace Research and Security Policy told DW: "There is no fixed definition." Publicly, it is not often clear why someone is classified as a threat, he said.

Why was Amri not under constant surveillance, although he was considered a threat?

Security services are under significant criticism for losing track of a known criminal and Islamist. The Federal Criminal Police Office (BKA) lists 549 people in Germany as threats from the Islamist scene. That may seem low, however round-the-clock surveillance is difficult: Every person requires between 25 and 40 security personnel. "You can't do that on just any kind of suspicion," Kahl said. "There has to be compelling evidence."

Kahl admonished the criticism: "He was under surveillance: His telephone was tapped and they were gathering information on him. But when nothing valuable surfaces after months, which appears to be the case here, you stop at some point."

Investigators could not build a case against Amri. His deportation had already been in the works, but it was delayed. Then he vanished. "Of course, you can ask how he ultimately slipped away from surveillance. However, the police and security services themselves don't agree on the reason why," Kahl said.

Why are authorities unable to stop such people?

Cooperation among agencies at state and federal levels is legally and in real terms, complex. "There are many rules governing how agencies cooperate - what they are and are not allowed to do," Kahl said.

It becomes even more complicated at the EU level. Relevant information is often overlooked or lost. Surveillance requires a high legal standard. Intelligence services are overseen by a parliamentary committee, said Thomas Grumke of the University of Applied Sciences for Public Administration of North Rhine-Westphalia. "That is both the boon and bane of the rule of law."

Surveillance poses practical shortcomings as well, such as the difficulty in finding interpreters who understand uncommon spoken dialects of a language.

Amri served four years in Italian prison before coming to Germany in 2015. Was this not enough to prevent his entrance?

"His past criminal record is not a cause for detention. He already served his sentence," Kahl said. "He was also known to be connected to Salafists. That is not enough to detain someone." It was difficult to stop his onward travel once he was within the Schengen Zone. However, he would have to have been returned to Italy to process his asylum request, in accordance with the Dublin Regulation.

What crimes are grounds for deportation?

"Deportation is not possible if a person faces death or torture in his native country, regardless of what he has done here," Kahl said. Only convictions of high crimes can lead to deportation: threat to life and limb or to sexual self-determination. A prison sentence of one year or more is cause for "urgent deportation." A prison sentence of three years or more is grounds to deny refugee status.

This does not automatically translate into deportation. As with Amri, home countries often refuse to take back the convict. In October, Interior Minister Thomas de Maiziere put forward a bill that would make deportation of those who meet the criteria a threat easier. "Foreigners required to leave who have been sentenced for crimes considered to be a considerable risk" should be more easily transferred to immigration detention, according to the draft.

What can be done when countries like Tunisia do not take back deportees?

Legally speaking, Germany cannot force a country to accept someone. There are political means, however, such as "no longer receiving support from us and that damages our relationship," Kahl said. "Or revoking promised economic relief."