Terrorism suspect Jaber al-Bakr, who was found hanged in his Leipzig prison cell this week, first came into contact with radical Islam in Germany and become a member of the Islamic State extremist group, according to German reports on Friday.

The weekly Welt am Sonntag reported that US intelligence service officials had told their German counterparts that they had intercepted several phone calls between the 22-year-old Syrian refugee and a contact person in the Islamic State group in Syria.

Al-Bakr had reportedly discussed during the calls with his Islamic State contact plans for a terrorist attack, which the German authorities say included an assault on a Berlin airport.

At the same time, the news magazine Spiegel quoted Alaa al-Bakr as saying his brother was radicalized in Germany, where he arrived in February last year before obtaining temporary asylum.

A Berlin-based imam was reportedly a key influence on al-Bakr, who the brother accused of brainwashing al-Bakr and ordering him to fight in Syria.

Al-Bakr joined Islamic State in Syria in September 2015, according to the report.

German Chancellor Angela Merkel called for an inquiry into al-Bakr's suicide following a political scandal unleashed by his death.

When a suicide occurs in a jail "then something has gone wrong and early warning signs have not been recognized resulting in misjudgments being made," said Merkel's spokesman Steffen Seibert.

Merkel's call for an inquiry could possibly lead to her into a clash with her conservative party ally - the premier of the eastern German state of Saxony, Stanislaw Tillich - who has strongly ruled out an independent investigation.

An autopsy confirmed that al-Bakr hanged himself from the bars of his prison cell using his T-Shirt.

Tillich admitted on Friday that mistakes had been made.

"In any event, the suicide should have been prevented," Tillich told the upper house of the German parliament, the Bundesrat, which represents the country's 16 states.

"Dealing with a prisoner accused of terrorism was not handled in [the] necessary way," he said. But "the establishment of an independent investigation commission is opposed by both the Saxony state government and myself."

Tillich's handling of the al-Bakr case was "an absolute scandal," political scientist Hajo Funke told dpa. "A broom is needed to clean all this up."

Federal Interior Minister Thomas de Maiziere has also called for an investigation into the suicide of al-Bakr, who the authorities claim was planning an attack with "an Islamic State context" against a Berlin airport.

The Federal Prosecutor's Office said on Friday that al-Bakr had refused to make any further statements after his first police interrogation and after being questioned by a judge in Dresden, the capital of Saxony.

Siebert said al-Bakr's death made the investigation into those behind his alleged plot that much more difficult, echoing remarks made by de Maiziere, who said described al-Bakr's death as a blow to the fight against terrorism.

The justice minister in Tillich's Christian Democrat-led government, Sebastian Gemkow, has repeatedly rejected suggestions that he should resign in the wake of al-Bakr's suicide just two days after his arrest.

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