Prosecutor said it means he may now be tried for murder in an adult court

But the country's Migration Agency has said he is actually older than 18

A Somalian migrant alleged to have stabbed a Swedish social worker to death and thought to be aged only 15 is now said to be at least three years older, it has emerged.

Alexandra Mezher, 22, was killed after she tried to break up a knife fight at an adolescent migrant centre where she worked in Gothenburg, Sweden.

Police say she had intervened when Youssaf Khaliif Nuur launched an unprovoked attack on another resident at the centre, suffering fatal knife wounds to the back and thigh.

Sweden's Migration Agency claims Youssaf Khaliif Nuur (pictured in court) is now believed to be 18 or older, rather than 15 as it was initially thought

Killed: Alezandra Mezher, 22, was stabbed to death when she stepped in to stop a fight at the adolescent migrant centre in Molndal, Gothenburg, where she worked

Argument: A person in police custody had allegedly attacked another person at the housing centre (pictured) and Miss Mezher was trying to break up the fight

Psychology graduate Miss Mezher had warned her mother she ‎was caring for 'big powerful guys' aged up to 24 in the shelter for children aged 14 to 17.

Johanna Mahlen, a press spokesperson for Sweden's Migration Agency, told The Local 'the applicant did not make it probable that he was under 18' when he applied for asylum in Sweden.

It follows media reports that Nuur had applied for a work permit in Sweden, despite the obvious fact teenagers do not leave school until they are 18.

Linda Wiking, the case prosecutor, told local paper Goteborgs-Posten he may now be tried in an adult court.

'The migration agency's decision that he is an adult is also relevant in the criminal case,' she said.

Speaking to MailOnline, Miss Mezher's mother Chimene said it was hard to describe how she felt at a time when she was still coming to terms with her daughter's death.

'But I do however think its a good thing if he is tried as an adult and it would be for the best if he is sentenced to spend a long time in prison because what he has done is unforgivable,' she said.

'I'm a mother who has lost her daughter. That is the worst thing that could happen to me and the family. He has destroyed our whole life.'

My family does not feel well, I do not feel well, everything is dark right now: it is like living in a long and terrible nightmare.'

But they will still have to wait for a final decision on his exact age - something which will make the difference between getting as little as two years in jail, Professor Jerzy Sarnecki, of Stockholm University's department of criminology, told MailOnline.

'If Yuseef is a minor and only as reported 15 years old, his time in prison will be reduced severely,' he said.

'Minors in the age between 15 to 17 are rarely sent to prison in Sweden. But when someone is convicted of such a serious crime as murder, I would expect that he will get a prison sentence - but the verdict will be much milder then if he was an adult.'

Professor Sarnecki said, if found guilty, Nuur would be sent to a special facility for under-18s, serving a maximum sentence of four years.

However, if he is convicted of manslaughter instead of murder the sentence could be half that - and he would more than likely be allowed to remain in Sweden afterwards.

Young: The 22-year-old psychology student, pictured at her university graduation in June last year, suffered two knife wounds to her back and thigh and died from her injuries in hospital

Tragedy: A police officer in Sweden has told MailOnline the psychology graduate (right) died trying to break up a fight between two migrants

Even if they do try him as an adult, for which the sentence would be much harsher, he may be allowed to stay.

'If there is a risk that the defendant might get killed, tortured or harmed in any other way in his home country, which could be the case when the defendant is from Somaila,' Professor Sarnecki.

'It depends from what area of the country he is from. If he is from an area controlled by Al Shabaab and not the government, the expulsion will be cancelled after he has served his time in prison and he stays in Sweden.'

But getting a definitive answer on his age looks likely to prove difficult.

'The Swedish courts knows that the scientific ways to determine ones age always leaves a bit of uncertainty,' the professor said. 'There are no absolute methods and the court always needs at least two or three opinions from different branches of the government to be sure.'

Last month a source at Gothenburg Police told MailOnline the alleged murder had begun with an attack on another boy.

‘There was an argument between the suspect and another resident at the hostel,' the source said. 'I don’t know the reason for the argument between the two residents.

‘But in some way this woman got between them. She was trying to break up the fight. Miss Mezher tried to separate them and told the suspect to stop but then he [allegedly] stabbed her.’

The source dismissed any suggestion that the killing had a sexual motive.

He added murder suspect Nuur had had complained of feeling 'unwell' the night before and had not slept at all in the hours before the alleged attack.

It has been suggested that Nuur has psychological issues and had a breakdown in the hours before the attack and complained of hearing voices in his head.

The source revealed: 'The knifeman may have had some kind of breakdown. He may have suffered some trauma previously in his life.'

Court papers: Youssaf Khaliif Nuur appeared in court in Gothenburg last week charged with murder and attempted murder

The source said he was heard pacing around the his bedroom for most of the night before the attack at 8am on Monday.

The Gothenburg Police source told MailOnline: ‘There was no sexual motive in the murder. This Somali boy did not have a motive for the attack other than he was not feeling well.

‘This lady tried to separate them and told him to stop but he stabbed her. The suspect had been up all night. He had said he was not feeling well the night before.’

Managers of the child refugee centre should not have let Miss Mezher work the night shift on her own if they knew one of the residents was suffering a mental health breakdown, the Gothenburg Police source claimed.

He told MailOnline: ‘If the management of the centre knew that one of the residents was not [mentally] well should have got in extra staff to help her that night.

Nuur appeared at Gothenburg District Court last week charged with one count of murder and one count of attempted murder.

Locked up: He is being held at the high security psychiatric clinic at Sahlgrenska University Hospital, Gothenburg. The facility is for patients with psychotic illnesses

The source, who has worked for Gothenburg Police for over 30 years, has detailed the ‘explosion in crime’ in the Swedish city since the country ‘threw open the doors’ to mass immigration.

He told MailOnline: ‘We have experienced an explosion in crime, particularly violent crime, since our politicians threw open the doors to immigrants from all over the world.

‘I have worked in Gothenburg for more than three decades and I have never known anything like it.

‘The city always had a bit of trouble, but it was usually amongst people who were drunk and knew each other.

‘But now I will not let my children go into the city after 2pm and especially not at night.

‘There are violent gangs roaming around the streets and the use of knives and other weapons have become normal, rather than a rarity as it used to be.

‘There are 20 robberies in Gothenburg every day - usually with violence or the threat of violence.

‘This week an 83-year-old lady had a gun put in her face when she was mugged on her way to play bingo. They got away with just 50 Kroner [about £4].

‘In the summer there were mass shootings with lots of people killed, including a little girl.’

He added the city streets were now at the mercy of gangs of young men mainly from North Africa.

The source revealed: ‘In the past couple of months we have noticed a dramatic increase in street robbery. This is a new phenomenon.

NEW POLICE CHIEF SAYS MIGRANTS RESPONSIBLE FOR COLOGNE CHAOS Revellers at Cologne's New Year celebrations set off fireworks during the chaotic party in the city centre Cologne's new police chief has said the New Year attackers in Cologne were migrants, adding many were from countries where such sexual assaults may be more common. Jurgen Mathies told The Local that planning of the attacks was transmitted via social media, with people telling one another they were heading to Cologne city centre for a 'party'. He acknowledged many of the attackers were migrants - saying they were from countries where they might be familiar with such assaults - 'where women are hemmed in and then abused by a large number of men at once'. He added: 'I must say that this phenomenon was not known to me in Germany before.' Police in Cologne have been struggling to restore public confidence after hundreds of women said they were groped and robbed in a mob of mostly North African and Arab men in chaotic New Year festivities. Reports filed by officers after the event described a scene of lawlessness. Officers working in the city centre were swamped by upset women claiming they had been groped and robbed, with many forced to walk a 'gauntlet' through packs of drunk men. The event triggered massive debate in Germany about the ability of the country to assimilate the more than one million asylum seekers that arrived last year. Advertisement

Hard working: Psychology graduate Miss Mezher who was a Lebanese Christian whose parents were from Beirut, was today described by her mother Chiméne as an 'angel'

Candles and floral tributes fill a table at a memorial service for Miss Mezher in her home town of Boras, some 40 miles from Gothenburg, on Wednesday

‘They will do anything to get money – steal a person’s wallet, iPhone, jewellery. And then they will attack their victim and kick them half to death.

‘Drug crime has gone through the roof. Every drug is available on the street nowadays, even in broad daylight.