German authorities are struggling to explain how a terror suspect accused of planning to blow up a Berlin airport was able to kill himself in his prison cell.

Jaber al-Bakr, a 22-year-old Syrian with probable links to Islamic State who was arrested on Monday morning following a 48-hour manhunt having been handed over to police by three fellow Syrian refugees, hanged himself with a t-shirt in his Leipzig prison cell on Wednesday.

The interior minister, Thomas de Maizière, said “urgent questions” had to be answered as the government of Angela Merkel – already under pressure amid a growing perception that its refugee policy is putting internal security under threat – faced calls to explain the apparent failings of the country’s police and judiciary.

Rolf Jacob, head of the Leipzig detention centre, said checks on Bakr’s cell by prison guards had been reduced from 15-minute to 30-minute intervals, after a psychologist who had no experience working with terror suspects, said there were no acute signs Bakr intended to kill himself. He had been on hunger strike since his arrest.

“If there had been warning signs he would have been moved to a specially constructed cell,” Jacob said. Such a cell would have had padded walls and basic toilet facilities and he would have been given special clothing, he added.

Instead Bakr, who had had limited access to an interpreter, had remained in a standard cell. Two incidents in his cell on Tuesday involving damage to equipment had been explained as vandalism rather than signs of a suicide attempt, Jacob said.

The last check on his cell was carried out at 7.30pm on Wednesday; the next was due at 8pm. But according to Jacob, a trainee prison guard decided “out of her own sense of duty” to take another look at 7.45pm, when she found Bakr hanged. He was pronounced dead half an hour later.

Jacob said his team had “followed to the last detail the letter of the law” and prison regulations would not persuade him to act differently if faced with the same situation again.

Alexander Hübner, the lawyer assigned to Bakr, called the case a “scandal of justice”, adding that his client should have been “Germany’s best-guarded prisoner”. The evidence of damage to equipment in his cell should have been signal enough, he said, that Bakr ought to have been on close suicide watch.

His death leaves in tatters an investigation into his plans to blow up a Berlin airport wearing a suicide vest made of TATP, a highly volatile explosive used in the attacks in Brussels and Paris. About 1.5kg (3.3lb) of the explosive was found in Bakr’s flat in the eastern city of Chemnitz. Experts say about 200g of it is enough to cause substantial harm.

The fact that they had managed to capture Bakr alive was seen as a victory for the German security forces and offered hope that he would divulge information about his plans including who had backed him. The two perpetrators of two other terror attacks in August, both of which are believed to have been Isis-motivated – a failed suicide attack at a music festival, and a knife attack on a train – both killed themselves before arrests could take place.

The general state prosecutor for the state of Saxony, Klaus Fleischmann, said Bakr’s death dealt a severe blow to the investigation. “It would have been a very nice angle to our investigation if al-Bakr – I’ll formulate it rather crudely – had come clean,” he said. “But we don’t know whether he would have divulged anything or not, and neither do we know whether he had any backers.”

Armin Schuster, the interior affairs expert for the Christian Democrats, said Bakr’s death should have been prevented.

“We have never before been so close to finding out the details of an [Isis-controlled] attack, so this [suicide] should have been avoided at all costs,” he said.

As a result of Bakr’s suicide, a 33-year-old man suspected of offering him support who is in custody in Dresden has now been put on round-the-clock surveillance, with someone posted permanently at his cell door.

It is not known when Bakr, who arrived in Germany as a refugee from the outskirts of Damascus in early 2015 and was registered as a refugee in June last year, might have been radicalised, but he had been under surveillance by intelligence services for several months. Police were working on the assumption he was in close contact with Isis. It has emerged that Bakr had travelled back to Syria since arriving in Germany, and had spent several months in Turkey, before returning to Germany with large amounts of money.

A police surveillance team was sent to watch his flat on Friday. Noticing their presence, Bakr fled from his flat and, despite warning shots fired by police, managed to escape to Leipzig. There at the railway station he met three fellow Syrian refugees who offered to put him up in their flat, apparently unaware he was a terror suspect until they saw “wanted” messages sent out by police in Arabic on social media sites.

The men tied Bakr up on their sofa and alerted the police who arrested him in the early hours of Monday morning. A picture of one of the men holding Bakr in a headlock, his ankles tied with a cable, has since been released.

There have been calls for the men to be awarded German medals of honour for effectively thwarting the attack, which investigators believe was to have been carried out within days.

One of the men identified only as Mohammed A told the German tabloid Bild that in return for giving him refuge, “he offered us lots of money, saying he’d received $10,000 from Isis, and that once he’d deposited the suitcase at a German airport he’d get more, and then he’d give us as much as we wanted”.

Bakr subsequently attempted to implicate the men during initial questioning, saying they had been involved in the terror plot. Police say they are investigating the claims.

The police had already come under fierce criticism over the manhunt, including why it was that Bakr was aware he was being watched, how he managed to escape, and why, when neighbours from a former flat where he had lived had called police to say he had returned asking for help, they took an hour to turn up, by which time he had long since left.

• In the UK, the Samaritans can be contacted on 116 123. In the US, the National Suicide Prevention Hotline is 1-800-273-8255. In Australia, the crisis support service Lifeline is on 13 11 14. Hotlines in other countries can be found here.