A suspected terrorist cell of the jihadist group "Islamic State" (IS) already had more detailed plans than previously thought for an attack in the western city of Düsseldorf when its members were arrested more than half a year ago, the German Federal Court of Justice (BGH) said in a ruling on Thursday.

According to the ruling on extending the period of investigative custody for the four Syrian men, they had planned to carry out the attack on a Friday or Saturday, "because the old city center of Düsseldorf is particularly busy on these days."

Their plans envisaged having two suicide bombers blow themselves up in the city center on two parallel streets, the ruling said. Two other terrorists were then to take up position at the four exits from the old part of the city to "shoot as many fleeing people as possible before also blowing themselves up when their magazines were empty."

Instructions in Raqqa

The cell was discovered after one of the men, Saleh A., confided in authorities in Paris in February, 2016. The information he provided allowed the other three men to be arrested on June 2 in refugee shelters in the German states of North Rhine-Westphalia, Baden-Württemberg and Brandenburg. The men are said to be aged from 25 to 31.

The four terror suspects remain in custody

Thursday's ruling also said that Saleh A. was commissioned with the attack and given detailed instructions for it at a meeting in Raqqa in Syria, where several IS leaders were present. Raqqa has become the de facto capital for the jihadist group in Syria.

At the meeting, a German IS member who had once lived in Düsseldorf was said to have given Saleh A. a sketched map of localities there.

The final details of the attack were to have been discussed in the southern German city of Heidelberg, the ruling said, but by that time Saleh A. had already handed himself in.

Security flaws in Berlin attack

The new revelations come as Germany continues to analyze the security weaknesses that allowed the Tunisian Anis Amri to slip through the gaps and carry out a deadly attack on a Christmas market in Berlin in December.

The interior minister of the western state of North Rhine-Westphalia, Ralf Jäger, on Thursday came under vehement criticism from the conservative opposition at a meeting of the internal affiars committee.

Amri had been monitored for many months

Joachim Stamp, from the Free Democrats (FDP), accused Jäger of "pushing away" responsibility for failures on the part of security authorities, which monitored Amri for several months before deciding that he did not pose a concrete danger.

"We cannot allow this continued irresponsibility any longer," Stamp said, describing Jäger as "no longer qualified to investigate" the circumstances surrounding Amri.

The FDP parliamentary party had previously called for Jäger's resignation, as had the Pirate Party. The Christian Democrats (CDU) had joined in criticism of Jäger, but has not yet called on him to step down.

'Wrongly assessed'

Jäger admitted that Amri, who spent some time in the state, "was wrongly assessed." But the interior minister pointed out that all 40 federal and state security authorities represented in the Joint Counter-Terrorism Centre (GTAZ) had come to the conclusion that Amri did not have any concrete plans for an attack.

He emphasized that "analysis must come before the question of who is to blame."

Before the truck attack in Berlin, in which 12 people were killed and dozens more injured, Amri often traveled between North Rhine-Westphalia and Berlin. He had been on the watch list of German security authorities since 2015 and was a regular subject of discussion, but was finally dismissed as being merely a petty criminal and drug dealer.

Amri was shot dead by a police patrol in Italy after several days on the run.

German authorities are also under pressure after revelations that three Islamists seen as a security risk had fallen off the radar. Nearly 550 people in the country are considered to pose Islamist threats.

tj/rc (dpa, BGH)