All luggage was then confiscated for 'checks' by American Airlines and Heathrow authorities, but passengers were kept in the dark as to the motive for doing so.
Up to six cabin crew members and two passengers, including a man said to be in his 60s and another in his 40s, are believed to have felt faint some 1,600miles into the 5,500mile flight.
American Airlines 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.'
Aviation expert and pilot Bruce Rodger told MailOnline that the captain did the right thing by turning the flight around.
"Anytime there is a problem in flight when passengers are ill - especially five or six - the captain will most likely turn the plane around to land it at the closest place," he said.
Rodger, who has over 20 years of flying experience especially with several different Boeing 700-model type of planes, explained that since the flight was in it's early stages of journey that food poisoning is not likely a factor as to what caused people to get sick.
Instead, he said that in this case, air quality could be a question.
"Air quality on the air plane is one thing I would consider. This situation could also just be circumstance," Rodger said.
"It sounds like it was bad and this could have been a situation that the air quality was possibly compromised somehow.
"The air on that type of plane is completely clean for passengers and they are always getting clean quality air."
The airline said last night that passengers were being reunited with their bags, but a spokesman would not reveal 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."
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.
"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."
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 Heathrow at midday on Wednesday and was due to arrive in LA later that day.
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.
She added: "Everything was checked and tested but there were no readings for any substances of any sort that may have been a problem."
The spokesperson said firefighters had 'no idea' what may have caused people to fall ill.
The London Ambulance Service tweeted: "We have attended an incident at #HeathrowAirport today. We checked over six patients who were feeling unwell. They were discharged on scene."
Earlier on Wednesday, a spokesman for American Airlines said the plane had turned around after a medical emergency and that it was not security-related.
The spokesman added: "American Airlines Flight AA 109, a Boeing 777-300, operating to Los Angeles International Airport (LAX) from London Heathrow, is returning to Heathrow due to a medical emergency.
"The aircraft departed London Heathrow at 1205 local time and is expected to land at Heathrow at 1700 local time. We apologise to our customers for the inconvenience to their travel plans."
It has been suggested by one man on board that the passengers and cabin crew 'suffered loss of equilibrium', though this has not been confirmed.
Lee Gunn tweeted: "Several crew and several passengers suffering with equilibrium, scheduled time of arrival in LHR is 5pm @flightradar24."
Equilibrium loss during air travel occurs when the air pressure in the middle ear fluctuates due to a change in altitude or imbalance in cabin air pressure.
Passengers were put up in a hotel last night and it is understood many will be found a seat on another flight to Los Angeles later today.
- Daily Mail
