Panicked passengers have told of the scary moment a flight attendant fainted and up to five other cabin crew members mysteriously fell ill on a Los Angeles-bound flight from Heathrow.
Those on board the American Airlines flight have described how they were then kept in the dark about what was happening as the aircraft declared an emergency and returned to London.
Up to three passengers are also believed to have begun feeling faint, including a man said to be in his 60s and another in his 40s.
Initial tests failed to reveal what caused some of those onboard to fall ill, while mystery surrounds why everyone's luggage was later seized for mysterious 'checks'.
American Airlines said tonight that passengers were being reunited with their bags, but a spokesman would not say what checks had been carried out or whether anything had been found.
He added: 'American Airlines, and Heathrow authorities, were inspecting bags and cargo', before revealing that the airline's maintenance team was now doing a ' thorough inspection of the aircraft.'
The airline has so far refused to speculate on what may have caused people to feel unwell, saying only that 'paramedics evaluated passengers and crew. None requested further medical attention.'
Alan Gray, 41, who told of how flight AA109 was given an escort by emergency services as it taxied to the terminal, said he and his fellow passengers had not been given an explanation about why their bags were confiscated.
The 777W is surrounded with medical teams and fire engines. Medics coming aboard. pic.twitter.com/hG5WPQTVS1— Eric Winter (@elwinter) January 27, 2016
'They wouldn't let us have our luggage. They're doing checks on it so it looks like there could be something more to it,' the band manager told MailOnline.
'When we landed there were police cars, ambulances and firefighters who escorted us down the runway and then we were held for 45 minutes before docking.
'Eventually, when we did dock, there was only one person who came on and he was monitoring the air.
'Then the paramedics were allowed onboard to treat those who were ill and everybody was let off.'
Earlier today it emerged the American Airlines flight turned around mid-air some 1,600miles into the 5,500mile flight.
Mr Gray was onboard the flight with boyband Race the Horizon, who were finalists on Britain's Got Talent in 2012.
One of the members, Kris Evans, 25, told MailOnline: 'It was just a bit mad. We didn't really know what was going on - initially we thought it was to do with the air conditioning.
'A cabin crew lady fell within touching distance of where we were sitting and then another guy next to me looked a bit iffy.
'There were around three passengers who fell ill.'
Mr Gray added: 'One of the flight attendants was walking down the aisle when she collapsed. Then up to six other cabin crew members said they felt light-headed and as though they were going to faint.
'It was at that point the captain said he was turning the plane around. He said he wasn't willing to take the risk to keep going and hadn't got the crew to do it.
'Then three passengers became really unwell too, and a few others were turning a bit pale.
'It was a bit strange we didn't just land in Iceland.'
Meanwhile, another passenger Lee Gunn told Mirror Online: 'About 2.5 hours into the flight just as we were passing Iceland we had a Tannoy announcement asking for any doctors, nurses or medical professionals on board to report to the boarding doors to assist with unwell passengers.
'The lights then came on in the cabin and there was lots of commotion.
'It was also reported that 7 of the crew - 13 on board in total I believe - had fallen ill, along with "many' passengers".'
The AA109 flight left London Heathrow airport at midday and was due to arrive in LA later today.
The Boeing 777-300 was close to Keflavik, Iceland, when it returned to its origin, some four hours into the flight. It landed in London shortly after 5pm.
A specialist team from London Fire Service was then on hand to carry out tests for 'elevated levels of any substances', a spokesperson said.