The passengers queuing for British Airways flight 77 from Heathrow comprised the clientele that might be expected to board a flight bound for a mineral-rich African country.

Many of those waiting at Terminal 5's Gate A18 at 7.40pm on Tuesday night were expatriates – including British, Canadian and American engineers heading out to work in Angola's lucrative oil fields.

Only one passenger, Jimmy Mubenga, was dreading his arrival at Luanda airport.

Within 50 minutes, his muscular 6ft body would be laid out along an aisle at the rear of the plane, seemingly lifeless, as the aircraft was diverted from the runway and returned to the stand, where paramedics were waiting.

Mubenga's last 50 minutes alive were tonight under investigation by detectives from Scotland Yard's homicide squad. The death of the Angolan father of five while he was being deported, after losing a legal appeal to remain in the UK in August, is being treated as unexplained.

The police investigation is likely to focus on the actions of three private security guards who attempted to restrain Mubenga, an apparently healthy 46-year-old judo enthusiast.

Detectives have already questioned passengers on the flight, four of whom have given detailed accounts to the Guardian. While some were reluctant to give their full names, fearing for their safety in Angola, they have described how Mubenga began resisting his deportation, prompting the guards to restrain him.

In apparent distress, he was heard to say: "I can't breathe, I can't breathe" for about 10 minutes before passing out. Two witnesses recalled him saying: "They're going to kill me."

"For the rest of my life I'm always going to have that at the back of my mind – could I have done something? That is going to bother me every time I go to sleep," said Michael, 51, a US citizen. "I didn't get involved because I was scared I would get kicked off the flight and lose my job. But that man paid a higher price than I would have."

Details about the chronology of events on BA flight 77, which had been scheduled to depart at 8pm, are incomplete. What is clear is that when Mubenga received a call from his wife, Makenda Kambana, at 7.30pm, he appeared calm.

He was sitting at the rear of the aircraft, between two guards, with a third in the seat in front of him. The guards worked for G4S, a private security firm contracted to escort deportees for the Home Office, which has declined to comment on the case or answer detailed questions put to it by the Guardian.

Makenda had talked to her husband earlier, while he was in transit to the airport around 3pm, and he had appeared to be calm and getting on with his guards. "He was friendly with them. They did not put him in handcuffs because he was good to them. I heard them asking him how are the children."

When she received his call from the aircraft, Mubenga sounded dispirited but calm. She recalled: "He was saying: 'I don't know what I am going to do, I don't know what I am going to do'. Then he said 'OK just hang up and I will call you back'."

Shortly after that call, a confrontation broke out with the guards. Witnesses said Mubenga was shouting that he did not want to go home. He was physically and resisting deportation.

His security escorts are likely to have known that in 2006 Mubenga was convicted of actual bodily harm after a brawl in a nightclub, for which he served a two-year prison sentence.

Passengers who entered the aircraft around 7.40pm were met with the sight of four brawling men – some presumed the guards were police or air marshalls. They then described seeing the guards "on top" of Mubenga, forcing him on to or under his seat for anything between 10 and 45 minutes.

Ben, a 29-year-old engineer, saw one guard reach for his handcuffs to restrain Mubenga. Michael, standing nearby, said: "The first thing I saw was the stewardesses running forward. One of them was almost in spasms she was shaking that bad … I saw three men trying to pull [Mubenga] down below the seats. All I could see was his head sticking up above the seats and he was hollering out: 'Help me'."

Passengers were moved away from the rear of the aircraft, and into first class. "You could hear the guy [Mubenga] screaming at the back of the plane," said Ben. "He was saying: 'They are going to kill me'."

BA stewards are understood to have moved two women sitting in the row of seats adjacent to those occupied by Mubenga and the guards.

The vacated seats were taken up by Kevin Wallis, a 58-year-old engineer, who claims to have had a full view of the ensuing confrontation just a few feet away.

As Mubenga resisted, Wallis heard one guard say: "He'll be alright once we get him in the air." It was around this time – 7.50pm – that Wallis took at call from his wife at his home in North Yorkshire, who said the commotion in the background "sounds really nasty".

Wallis told his wife it was a deportation, and put the phone down. Wallis said he listened to Mubenga repeatedly complain that he was unable to breathe "for 10 minutes, at least" before he went silent.

"They [the guards] checked his neck pulse and his wrist pulse," said Wallis. "That is when they looked a bit worried."

Andrew, a 44-year-old from Eastern Europe sitting in row 28, recalled seeing two men pushing down on Mubenga, who was consistently calling for help. Andrew heard cries of "don't do this" and "they are trying to kill me". He added: "In the beginning his voice was strong and loud, but with the time passing by the voice was losing its strength."

Michael had a similar account, recalling Mubenga was saying "help me, help me" while three security guards were on top of him. "And then it went kind of quiet," he said. "The last thing we heard the man say was he couldn't breathe."

It is unclear how much of this information was being conveyed to the cockpit. Some time after 8pm, the pilot commanding BA flight 77 headed for the runway. But his aircraft would never get into the air.

At 8.25pm, police and paramedics were called to a man unwell on the aircraft, which was returned to the terminal.

The Guardian's four witnesses did not recall a PA announcement asking if a doctor was on board.

"They left him in his seat until the paramedics came," said Wallis.

"I'm not sure he got any attention from anybody until the medics got there and that was 15, 20 minutes after everything went quiet," added Michael.

"Maybe somebody could have revived him if they had been asked. I can give CPR."