The man leans forward in the video, a stethoscope dangling from his neck, and urges: “All the people in England, I ask you again, all the Muslims over there, leave the land of England and come to make hijrah [the journey] here. There is a great cause being fought here, the caravan is leaving.”

The speaker is Ahmed Sami Khader, 23, from south-east London, and the caravan left in the spring. It is almost four months since this newspaper revealed that he and eight other British-Sudanese doctors had crossed the Syrian border at Tel Abyad to join Islamic State and answer its call for doctors.

Since the Britons disappeared into Isis’s self-proclaimed caliphate, the Observer has collated testimony from family members, friends and former classmates – speaking for the first time, on condition of anonymity – in an attempt to reconstruct what happened to the five women and four men. Were they safe? Did they want to come home?

Hisham Muhammed Fadlallah, one of the nine British students believed to be in Isis-controlled territory. Photograph: Observer

Their accounts reveal high-level talks took place to rescue the nine, that they remain in contact with their parents, and that more young British medics than previously thought may have travelled from the Sudanese capital to join Isis.

They also indicate that all were indoctrinated during their studies at Khartoum’s private University of Medical Sciences and Technology by a seemingly benign Islamic organisation that friends claim was a well-oiled “base operation” for Isis to recruit young medics. More young doctors, they say, have been brainwashed by Isis and remain in Khartoum. Two weeks ago a second group of 12 medical students from the university, including seven Britons, arrived in Turkey in an attempt to cross into Syria.

So far, two brothers from Leicester have been intercepted on their way to the Syrian border and last week were returned to Khartoum. On Saturday night The Foreign Office confirmed it was working with Turkish police to try to prevent the remaining five Britons reaching Isis. At least 17 young British doctors are known to have attempted the journey from Khartoum to Syria. How many more are ready to follow?

“What is concerning is that the students who have gone are not the only ones recruited; there are reports of more students currently studying there who have been indoctrinated but have not been told to go,” said a sibling of one of the nine medics inside Syria.

But what was it that persuaded that initial batch of nine to abandon their families and to travel more than 1,200 miles north to serve a regime that has become synonymous with brutality?

Life for foreign students on the Khartoum university’s sweeping campus is austere by western standards. Extracurricular activities are few, an exception being the Islamic Cultural Association, a low-key group founded in 2006 to help western students become closer to their religion. The seemingly innocent organisation is accused of being the machine from which Isis has recruited at least 24 medical students. “It became a front for a recruitment operation that focused on recruiting into Islamic State,” said one of the medics’ siblings. The association became increasingly hardline after 2011, when Mohammed Fakhri, from Teesside, became its president.

Tasneem Suleyman Huseyin Photograph: Observer

Speaking from Khartoum, one former student described how the ICA regulars who joined Isis became increasingly distant: “You wouldn’t really hear from them, a few brief salaams from a distance.” Other control tactics were used. “They would slowly isolate the individuals from their original friends and family so that they only trusted and respected the inner circle.”

Light entertainment events became pious affairs. Open mic nights promoted by the ICA were, according to one ex-student, held in an “awkward atmosphere, women and men sat apart”. Applause was prohibited; poetry was celebrated with chants of “Allahu Akbar” [God is greatest].

Colleagues became concerned that elements within the ICA were promoting extremism. One friend of Lena Mamoun Abdelgadir, the 19-year-old medical student from Norfolk who was among those who joined Isis, said: “Some of the conversations between members of the inner circle depict heated arguments about how to kill sinners who have committed apostasy, adultery. Not if they should kill them, but how.”

Siblings of the nine believe the ICA – 1,202 Facebook likes at the time of writing – was targeted by Isis as a way to source young, impressionable but highly prized female medics. Peers say that women wearing the full niqab met every Wednesday outside the university’s Abdel-Kariem hall. They say Fakhri was often at such meetings, leading to speculation he might have helped radicalise the women. His father said that he knew nothing about such claims and refused to respond to claims that Fakhri was the first of the nine medics to visit Syria in 2014.

Another British Khartoum university student who also reportedly travelled to Syria last year, but is unconnected to the ICA medics, was charged in connection with a UK terror plot.

One theory among the families is that Isis targeted the medics long before the caliphate came into being. “The idea was that there is a pure Islamic state forming in the future and that there will be a signal given when they can go,” said one relative, who has investigated the ICA. Last summer, as Isis began calling for recruits to bolster its ranks, the ICA appeared to embrace Palestinian-American cleric Ahmad Musa Jibril, named by experts as the world’s most powerful recruiter for westerners looking to join Isis. During Ramadan last year, the ICA’s Facebook page hosted eight videos from Jibril over two weeks – now removed as part of a concerted response by the university, which promotes itself as secularist and progressive – that show increased monitoring and the introduction of student guidance sessions. The British embassy in Khartoum is in contact with the university administration “to address these concerns”.

The doctors crossed into Syria on 18 March and appear to have been taken to a sharia camp near Raqqa where new arrivals are taught the fundamentalist ideology of Isis. According to Abu Ibrahim Raqqawi, an activist with the group Raqqa is Being Slaughtered Silently, all nine Britons then attended a medical faculty adjacent to the Raqqa National Hospital. Isis opened the facility in January as part of its fledgling health ministry, with recruitment starting weeks before the British doctors arrived. A doctor at the hospital said the Britons joined “a lot” of young foreign medics, primarily Iraqis, Saudis, Libyans and Russians. An influx of African recruits had, according to Raqqawi, made it increasingly problematic to identify the Britons. In late April the group appears to have been split up throughout the caliphate. The doctor said he heard that three were to stay in the city, with the remainder split between the city of Al-Bab, east of Aleppo, Menbij’sNational Hospital, 12 miles from the Turkish border, and Deir ez-Zor, located deep in Isis territory and where Khader filmed his plea to English Muslims.

Rowan Kamal Zine El Abidine. Photograph: Observer

Clues to who is where are scarce. One of the nine, Mohammed Osama Badri, the eldest at 24, has resurfaced on social media and on 18 June posted an image of an Isis flag flying in a desert landscape that could be almost anywhere in northern Syria.

Details of the individuals connected to the initial group of British medics have started to emerge. Among the group is Osman Mustafa Fagiri, 23, whose Facebook messages describe walking the streets of Raqqa. Fagiri is understood to have joined Sudanese jihadists who went to fight in Mali in 2013. He visited Swiss Cottage in north London for 10 days last year before flying back to the Sudanese capital. Who he met in the UK and why are not known.

Then there is the mysterious figure of Fakhri, 24, who, according to Facebook, has links to both groups of British doctors. Sources claim that Fakhri, of Palestinian origin, visited Syria in 2014, which, if true, would make him the first of the British Sudanese doctors to join Isis. His father, who runs the Abu Bakr mosque in Middlesbrough, would not comment on his son’s whereabouts.

Lena Mamoun Abdelgadir. Photograph: Observer

For the families, the initial optimism that their children’s foray into Isis would prove short lived have faded. Parents have travelled to the border to await for updates from Turkish intelligence officials and the British authorities. Dr Maumoon Abdulqadir, an orthopaedic surgeon, has, according to relatives, moved almost permanently from the family’s home in the Norfolk village of Ashwicken to the Turkish city of Gaziantep, 18 miles from the Syrian border, to find out what has happened to his daughter Lena.

Some believe such gestures are futile. “Life has to go back to normal. We have other children, we have other commitments. We have to carry on with our lives and then continue efforts to bring these children out,” said one family source.

Nada Sami Khader. Photograph: Observer

Of some hope is the fact the families remain in contact with their children. Using only WhatsApp, the contact follows an eerily similar pattern: perfunctory in tone and lacking in detail. Parents receive messages routinely, varying from twice a week to every fortnight. “They send just one or two lines saying they are doing fine, that they are working in hospitals, doing their shifts, but they don’t give any details,” said one parent.

Another said: “They send one line: ‘We are OK.’ I am not sure if they are free to write, or the messages are written on their behalf. They don’t talk about their whereabouts, their intentions or if they want to come out.”

Other parents have received almost identical messages, fostering the suspicion that communication is censored. “All the parents are saying that it is not the same way their children normally write to them. The feeling has grown that they are different people,” said one. The consensus among parents is that Khader’s propaganda video was not voluntary. One set of parents became so exasperated that they asked their child to prove their identity. “They sent voice messages, in WhatsApp, just to reassure their parents.” Yet the voice sounded different.

“Even these voice messages have something wrong: the parents believe they are not behaving as they used to,” said the source. One parent stopped sending messages. Another said that the young doctors’ refusal to share their whereabouts suggests the WhatsApp messages are supervised.

Mohammed Osama Bedir Photograph: Observer

For now the advice given to the families is to maintain contact without antagonising Isis supervisors. “If family links are broken, it is Isis who will benefit,” said one family member in London.

At least one audacious mission to rescue the nine has been explored. In mid-April, according to a family source, Turkish and British security services examined the potential for a raid after receiving intelligence that the group was together in Raqqa. No details are known of the plan, but a special operations raid within the caliphate is obviously fraught with risk. “We heard that it was discussed but they didn’t go ahead,” said the source.

If the nine attempted to leave independently, one likely method would involve hiring a smuggler to navigate northern Syria, a perilous undertaking compounded by the counter-offensives of Kurdish forces pushing south. The current fighting has raised another grim possibility for the families. Not only do they risk being shot by advancing peshmerga troops, but there is a danger of being killed by US-led airstrikes targeting Isis positions. “Daesh [Isis] is coming under heavy bombing by the coalition and the Kurds. This situation makes us really frightened,” said the source in London.

Despite this, Isis smugglers are still getting through, recently guiding three British sisters from Bradford and their nine children to northern Syria.

The challenge for the nine doctors would be identifying mercenary smugglers they could trust to get them out. Raqqawi, describing roads littered with Isis checkpoints, said: “In Raqqa it’s impossible to get out. If they want to go back they have to contact civilians in the area, it is very risky. There are big walls between foreigners and local people, who are afraid to talk to them.”

The most likely scenario is a protracted waiting game, a hope that the caliphate implodes or that the medics become disillusioned by the increasing barbarism of Isis.

“They need to see the slow deterioration of the state, the behaviour of the people around them getting worse,” said a family source.

It remains unclear if the British female doctors such as Lena have been married. Although many female foreign arrivals quickly become “jihadi brides,” activists in Raqqa say that Isis’s foreign doctors are viewed differently, allowing them to work unmarried. The caliphate’s strict segregation rules – female medics can operate only on women – and a shortage of female doctors have amplified their worth.

But the families know that circumstances change quickly within the turbulent, embryonic caliphate. As the sibling of one medic said: “It’s not a small chance that none of them will come back alive.”