Germany's interior minister ordered increased security across the country after its terror alert showed a heightened threat of a terrorist attack. Many countries use similar warning systems, first developed in the US.
Police are increasingly patrolling train stations and airports
Terror alert levels reflect the existing threat of an imminent terrorist attack, though such danger can hardly be measured. The US Department of Homeland Security developed such an advisory system following the September 11 attacks.
Authorities raise the threat level to "high" - or in the worst case "severe" - if they receive pertinent information from their own intelligence service or other security authorities. They also rely on intelligence data from other countries they work closely with. This can involve tapped conversations, respective information in the internet or credible leads from informants in the jihadist scene. It is very rarely based on a concrete lead.
Members of German intelligence services recently questioned Islamists in Afghanistan for an entire week about terror scenarios in Germany and Europe. According to security authorities, some 40 Islamists from Germany are currently residing in Pakistan and Afghanistan.
At the same time, around 200 specialists from the Federal Criminal Police Office (BKA) and the Federal Office for the Protection of the Constitution (BfV) have continued to work on verifying whether the latest terror warnings from the US are true. This includes intensifying the surveillance of some 130 so-called "endangerers" living in Germany.
Various degrees of security measures
Normally, a collection of several indicators form a qualitative new threat situation. This can result in a concrete terror alert officially announced to the public, in turn generally leading to increased controls at critical points in a country's security structure.
De Maiziere said new, tangible intelligence has come to light
In Germany, the federal interior minister and his counterparts in the individual states are responsible for this move.
The scope of possible measures to resort to range from ordering increased passenger and luggage controls at airports, monitoring train stations and major road junctions to temporarily closing embassies at particularly vulnerable locations.
In the United States, the government set up the five-tiered color-coded threat level system to protect against future accusations that security authorities didn't take information about the imminent 9/11 attacks seriously enough and inform the public. Germany's interior minister Thomas De Maiziere probably had similar thoughts in his mind.
Last month, authorities discovered two US-bound parcel bombs originating from Yemen, one of which went through Cologne-Bonn airport in western Germany undetected. It turned out that Lufthansa had already tipped off the interior ministry about the terrorist threat from Yemen. The airline's warning reportedly reached De Maiziere already at the beginning of October - three weeks before the package bombs were found in Europe.
In recent weeks, German media have also repeatedly reported on alleged imminent terrorist attacks in the US and Europe.
In addition, there have been continual warnings by western intelligence services that terrorists could plan an attack still before Christmas - though the al Qaeda network has in the past liked to publicly spread such warnings for propaganda purposes.
Other governments have also reacted with official terror warnings to the supposed changed security situation. Germany is the third EU nation after Britain and France to announce such a warning to its population.
Author: Daniel Scheschkewitz (sac)
Editor: Rob Turner