17 years ago today, two passenger jets were hijacked and flown into the Twin Towers in New York City, in what has been described as the worst terror attack that ever took place on American soil.
In 2001, thousands of people died when a group of al-Qaeda terrorists launched attacks in New York, Washington, and Pennsylvania on September 11.
A group of 19 terrorists hijacked four airlines, flying them into the North and South towers of the World Trade Centre in New York; another was flown into the Pentagon; and another into a field in Pittsburgh Pennsylvania.
And, how does Donald Trump decide to commemorate the memory of this event 17 years later?
With this bizarre, to say the least, tweet:
17 years since September 11th!
Needless to say, Twitter users weren't buying it.
Many commented on how impressive it was that the president appears to be able to count.
Others highlighted how Trump appeared to brag about owning the tallest building in Manhattan just hours after the attacks.
Some responded with a memorial tweet from Barack Obama, which was actually honouring the dead.
Others argued that it was actually only a year since 'September 11'.
And others focused on how inappropriate the whole thing was.
There were also throwbacks to a particularly inappropriate arrival to a 9/11 memorial.