The Star-Spangled Banner was adopted as the official national anthem of the United States in 1931. Up until June 1942 there were no guidelines given as to how people were to behave during the playing of the anthem. Since then there have been a number of changes to the description of what people should do. For example, men were to remove their hats and women were to place their hand on their heart. Then it changed to instruct both men and women to place their hand on their heart when the flag was on display. Then it was changed again to instruct that your hand should be held over your heart whether or not the flag was displayed, essentially meaning for the entire duration of the anthem. To quote the source I've read (the Wikipedia article on the anthem):
Because of the changes in law over the years and confusion between instructions for the Pledge of Allegiance versus the National Anthem, throughout most of the 20th century many people simply stood at attention or with their hands folded in front of them during the playing of the Anthem, and when reciting the Pledge they would hold their hand (or hat) over their heart.
The Star-Spangled Banner: Customs and Federal Law
But it was the very next sentence that I found very interesting:
After 9/11, the custom of placing the hand over the heart during the playing of the national anthem became nearly universal.
It cites three sources directly after this sentence, however they are supporting references to the entire preceding paragraph. The links take you to legal documents, and none seem to support the change in custom after the 9/11 terrorist attack.
Is there any truth to this claim?
Edit: A commenter has linked a different Wikipedia article where another version of this claim is made:
According to the United States Code, those present should stand at attention with right hand over heart. It's noteworthy, this tradition only acquired widespread practice after the year 2001, ...
US national anthem protests
Again, even here it just gives links to the United States Code as a citation. None of it supports the claim about 9/11 or the year 2001.