Jaber Albakr was detained on Monday, two days after police discovered about 1.5 kg of explosives in his apartment.

A Syrian refugee arrested on suspicion of planning a major attack in Berlin spoke to a member of ISIS in Syria by telephone about a possible target a day before German police discovered explosives in his apartment, a newspaper reported on Saturday.

Jaber Albakr was detained on Monday, two days after police discovered about 1.5 kg of explosives in his apartment. He was found dead in prison on Wednesday. Authorities said he had committed suicide.

Germany’s Welt am Sonntag (WamS) cited investigation sources as saying US intelligence had provided a tip-off about Albakr after tapping several phone calls between him and an ISIS member in Syria. During the calls, 22-year-old Albakr spoke about his attack plans, the newspaper said.

In a call on Oct. 7, Albakr told his contact that 2 kg of explosives were ready and he named a possible target, saying a “big airport in Berlin” was “better than trains”, WamS reported.

In July, the militant group claimed responsibility for two attacks in the German state of Bavaria – one on a train near Wuerzburg and the other at a music festival in Ansbach that wounded 20 people.

WamS said federal prosecutors investigating the case assumed that Albakr wanted to make a vest packed with explosives for an attack.

The head of Germany’s domestic intelligence agency (BfV) has said Albakr was building a bomb and probably planned to attack one of the airports in Berlin.

Investigators said on Monday they believed Albakr was close to staging an attack comparable to those that killed 130 people in Paris last November and 32 in Belgium in March. They suspect he was inspired by ISIS.

Albakr arrived in Germany in February 2015 during a migrant influx into the country and was granted temporary asylum four months later.

The man who rented the flat in the eastern city of Chemnitz in which Albakr last lived – a 33-year-old Syrian who WamS named as Khalil A. – is in custody and is being investigated on suspicion of helping Albakr, the newspaper said.

Separately, German broadcaster ARD said Tegel airport in Berlin was possibly Albakr’s attack target. Without naming its sources, ARD said Albakr went to Berlin one weekend in the second half of September to spy out Tegel.

Bus tickets, among other things, proved that, the broadcaster said.

The Berliner Morgenpost newspaper and the regional broadcaster rbjib cited federal security sources as saying Albakr met a contact in Berlin while he was in the capital.

The newspaper Frankfurter Allgemeine Sonntagszeitung (F.A.S.) said train tickets that investigators found in Albakr’s possessions were key to uncovering the Berlin trip.

The federal prosecutor’s office declined to comment on media reports when contacted by Reuters.

F.A.S. cited a spokesman for the federal prosecutor’s office as saying there were not yet “enough links to IS that could be used in court.”

German security sources told Reuters that Albakr had traveled to Turkey after receiving asylum in Germany and spent several months there this summer.

F.A.S. said Albakr landed in the eastern German city of Leipzig at the end of August on his return from Turkey. The newspaper cited investigators as saying Albakr had already planned an attack at that point.

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