A woman lays her head on a row of names at the National September 11 Memorial, ahead of the 15th anniversary of the attacks in Manhattan, New York, September 10, 2016.

Two thirds of French people believe the truth about what happened on September 11, 2001, in the United States, has been deliberately hidden.

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The figures show that nearly three quarters of those under the age of 25 in a recent survey believe that the facts about what happened are being withheld.

This is in contrast to those over the age of 65, 55 percent of whom believe the same thing.

The research was carried out by the French polling firm Odoxa over the course of September 8 and September 9 and surveyed 1009 adults of 18 years and over.

As well as the two thirds that believe all the facts about the attacks were never made public, a further 45 percent believe that the real culprit behind the attack are not known and will never be known.

In fact, a large number of people believe that the official version of events is nothing but a “huge lie”, the report reads.

That said, not everyone thinks there is a conspiracy behind all this. A number of respondents believe that no one really knows who is behind the attacks.

This uncertainty, the report reads, has led a large number of conspiracy theories to spread.

As evidence of that, 28 percent of people believe that the American government had a role in the attacks, although what that role might be has not been explained.

The researchers add that this does not imply that 68 percent of people living in France believe in a conspiracy theory about the attacks, but simply that a large number of people think that governments don’t and can’t provide all the details about terrorist attacks.

The conspiracy theories arise, the research said, when elements of the official version are discredited whether those facts are the result of error or deliberate misinformation.

The United States marks the 15th anniversary of the 9/11 attacks today, Sunday, with solemn services to commemorate the victims of the deadliest terror strikes on US in 200 years.

Nearly 3,000 people were killed on September 11, 2001 when 19 Al-Qaeda suicide bombers hijacked four passenger jets, crashing them into the Twin Towers in New York, the Pentagon in Washington and a field in Shanksville, Pennsylvania.

The names of the dead will be read out in a remembrance service at Ground Zero in New York on the site of the rebuilt World Trade Center, and President Barack Obama will address a ceremony at the Pentagon.

"This weekend, we honor their memory once more. We stand with the survivors who still bear the scars of that day," Obama said Saturday in his weekly address.

Held at the September 11 memorial, the service will pause six times -- to mark the moments when each of the two planes hit, when each tower fell, as well as the attack on the Pentagon and Flight 93 that crashed in Pennsylvania.

During the first moment of silence at 8:46 am ET (14.45 CET), the time when American Airlines Flight 11 hit the north tower, houses of worship in New York have been asked to toll their bells.

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