Global Security Editor Rohit Kachroo pieces together what happened at Hospital Number One in the days after Russia invaded

Russian forces killed as many as 50 hospital patients in Mariupol, Ukraine , who resisted attempts to forcibly deport them - a calculated strike which the city’s mayor has called a “revenge attack” against citizens who were not sympathisers.

Invading troops stormed Hospital Number One in late March to investigate who was inside and where, according to an account of events compiled by ITV News based on dozens of interviews with eyewitnesses and officials.

The soldiers attempted to move groups of patients and staff to Russian-controlled territory around one week before a 500 kilogram high explosive ‘FAB-500’ warhead was dropped onto the intensive care unit where dozens of patients were being treated.

“It was revenge for disobedience,” Vadym Boychenko, the mayor of Mariupol told ITV News.

“It is a deliberate action. They deliberately killed people who were in this hospital.”

Although many of those now feared dead had refused to leave, some patients who might have gone had no choice because they were immobilised by illness.

There is no official account of what happened: many hospital records were destroyed, and some evacuees who shared their story with ITV News had an incomplete view of what was happening across the vast campus, because they were locked in the basement in the dark for days.

“It was pitch black when I was treating patients, I didn’t even know what day it was most of the time,” said one anaesthetist.

But when pieced together, their accounts indicate a Russian reconnaissance trip took place alongside an attempted mass deportation in advance of the deadly strike on 2nd April.

A senior western intelligence official said the attack is “evidence that Russia has a much clearer sense than people might assume about precisely who will be killed when it decides to attack.”

When Mrs Zubko, a 60-year-old resident of Mariupol’s Left Bank, was brought into Hospital Number One a few hours after the invasion began on February 24. The treatment she received was rapid and successful. She had been hit by artillery while at home, suffering wounds to her arms and legs.

A second victim, a man in his 50s, was injured in the same attack as he stood at a bus stop nearby. He too was saved and quickly discharged because the hospital still had power, water and was well-stocked with medicine.

The hospital campus is made up of eight units spread across a collection of buildings with the traumatology department, the largest, at the northern end of the site. By the end of the first week of the invasion the entire hospital system was under strain - every bed had been filled.

An ITV News crew filmed as injured people from Sartana, a small town on the outskirts of the city, streamed in after being hit in a series of shelling attacks by Russian forces trying to take the region. Queues at the hospital pharmacy were several dozen-long as supplies of medicine began to run out.

“For me the worst day was 27th February,” says Hrankov Yelyzar, an anaesthesiologist who fled as Russians seized his hometown.

“It was the day I treated my first child patient during the invasion. She was the first dead child we tried to help. We tried to raise her but we couldn’t do anything. I just cried, cried, cried. I could not take my face from inside my hands.”

Despite heavy shelling a short distance away, the hospital itself remained largely undamaged until mid-March.

“We were living in the hospital between 15th and 20th March and half of it was destroyed,” said one former member of staff.

Around March 21: The attempted deportation

Wrapped in warm coats and wearing thick hats, a 15-second video clip shows a row of people tiptoeing through the rubble of a destroyed building carrying rucksacks and carrier bags. Their faces cannot be seen but they appear to be moving in an organised and orderly fashion towards an armoured vehicle.

The footage was posted by Ukrainian authorities which claim it shows doctors and patients being forcefully deported from another hospital in Mariupol, Hospital Number Four. It confirms that the tactic, seen in other parts of the city, was being used by invading forces inside medical facilities.

Eyewitnesses and officials tell ITV News that Russian troops entered the hospital during the final days of March to force staff and patients to move to Russia or to Russian-controlled territory within Ukraine. Some resisted.

“Doctors did not obey, did not agree on those conditions that the criminal authorities and Russian troops demanded,” says Mayor Boychenko, whose account is supported by other witnesses who spoke to ITV News.

The deportation of Ukrainian civilians to filtration camps in Russia is a common practice used by invading forces. But Western officials believe that in Mariupol, invading troops were more discriminate than they were elsewhere, prioritising doctors over patients when they launched so-called ‘evacuation operations’.

Footage shows patients and doctors being moved from another hospital in Mariupol as part of forced deportations by Russian forces

The testimony of medical professionals is likely to be considered highly credible by war crimes prosecutors compared to patients whose recollections might be discredited due to the impact of medication, illness or injuries. So the move to remove doctors is thought to have been a key part of Russian attempts at a cover-up.

Some staff who spoke to ITV News did not see a compulsory evacuation take place, suggesting any operation was limited to only parts of the hospital campus. But others said that during the final week of March they saw the hospital being searched and assessed by soldiers.

“Chechen officers came earlier when trying to clear out the territory,” says Lydia, a nurse who worked at the hospital until early April.

“They were intelligence or something, and they passed the information about civilian patients in the otolaryngology (ears, nose and throat) ward”.

Speaking from Russian-occupied territory, her evidence appears to confirm that the Russian military had recorded details of where within the buildings patients were taking refuge.

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Sergiy Mudryi, who served as a doctor at Mariupol Hospital during March before moving to Kyiv, says intelligence officers were being assisted by sympathetic members of staff to establish which parts of the hospital were occupied.

“They knew that there were people in the basement of the ward where they were themselves. They knew there was also a third basement of the neurology ward, and the fourth basement of the trauma ward - because they were in contact with a nurse who was in the basement of the administrative ward.”

But even some of those who might have accepted the Russian offer to move were unable to leave.

Dr Sergiy Mudryi says intelligence officers were helped by sympathetic members of staff to establish which parts of the hospital were occupied

Dr Olga Kononova, who fled to Zaporizhia, said: “First of all, there were patients who couldn’t walk or were walking slowly.

“Overall, in the wards of the hospital, patients were either barely mobile, or completely immobile and couldn’t walk independently. Also, there were their relatives, families with children, in every ward there was a melting pot."

On the morning of April 6, Mariupol authorities sent out a notice on Telegram calling for further restrictions on the sale of Russian oil and gas to help save the city. In the fifth and final paragraph was the first mention of a major attack on Hospital Number One.

“Russian terrorist forces dropped several heavy bombs on a children’s hospital and destroyed one of the buildings of the city hospital No 1,” it said.

'They bombed these buildings, these buildings start to burn and all people who were in this building - they burned too' - Doctor Hrankov Yelyzar

Mariupol was a communications black hole and officials from the local authority were operating from Zaporizhia. It had taken around four days for news of the attack to reach the outside world, and even then details were scarce.

Two intelligence sources say they believe a FAB-500 air-dropped bomb was used, a Soviet-designed 500 kilogram warhead, normally used against military infrastructure. The same device was dropped to destroy the maternity hospital in Mariupol the previous month, according to the head of Ukraine’s national police.

The traumatology department at Hospital No 1 was targeted, which included the unit where many intensive care patients were being treated. Staff and patients had been huddled inside the basement.

'It was revenge for disobedience,' Mariupol's mayor says

Another building, known as the Polyclinic, was badly damaged too. Many patients had been moved from other buildings across the site which had been damaged by previous, smaller, shelling attacks reported during the days beforehand.

It is unclear how many people were inside the building. Several people who worked inside the traumatology ward during the previous weeks have said around 50 would have been there when the bomb was dropped. ITV News has been unable to locate a single survivor.

“All floors and basement were on fire we almost suffocated from the smoke,” said an eyewitness who saw the destruction from a nearby building.

“There were many injured, wounded, those who could not get to their feet, schoolchildren, newborns.”

Satellite images taken after the attack and assessed for this investigation show the traumatology ward and other buildings completely destroyed.

It wasn’t until April 20 when that video was released by the Russian Investigative Committee showing the damage to the hospital.

Medical equipment can be seem among the debris in the destroyed buildings. Russian officials said the Ukrainian military was responsible for the damage, but ITV News has seen no evidence to substantiate that claim.

Doctor Hrankov Yelyzar concedes now that after seeing what happened next, he struggles to contain his intractable wish to do harm to Russian soldiers.

'I really hate them all. I really want to kill them [Russians]'

“I am angry, I didn’t know that I can hate anybody in my life like I hate Russian people now,” he tells me at his new home in Dnipro, 200 miles from Mariupol.

“I really hate them all. I really want to kill them I really want to destroy their life. I want to do this. But that same moment I understand that I’m a doctor.”

In an update to the United Nations Human Rights Council (UNHRC) on Thursday, High Commissioner Michelle Bachelet said Mariupol was "likely the deadliest place in Ukraine" between February and the end of April.

She told the council that by March 10 - just days after Moscow's troops began to close in on Mariupol - the city was "completely encircled" by Russian armed forces and affiliated armed groups.

"All hospitals able to receive injured civilians were damaged or destroyed, including the childcare departments of Mariupol hospital No 3," said the high commissioner.

"By the end of March, the damage and destruction coupled with the lack of electricity and medical supplies meant that hospitals had effectively ceased to function."

She said the destruction and damage to civilian objects in Mariupol raises "serious concerns about compliance with international humanitarian law".