The 911 memorial was built on the site of world trade center towers collapse. The memorial was finally finished in 2011. Ten whole years between the towers collapsing and a memorial being built. This delay is because of many factors, which include differences in ideals between the people involved in this memorial.
The motivation behind the attacks was all based on extremist ideals, which affect the memorial of the event. For example, there are historians that want for there to be a mention of the Arab-Americans that lived in the area long ago who were a positive influence in the United States history, but the people who made the memorial decided they didn’t want to include that information.
At around the same time it opened, another controversial construction was made nearby. Park51, while not a memorial, it’s plans to be built were met with similar issues. People thought because the terrorist that caused 911 were Muslim that this “mosque” shouldn't be built there. The owners don't consider it a mosque, instead they use “prayer space”. The controversy then becomes a matter of language since the same people become less opposed to a prayer space than they would a mosque.
Martin Luther King Jr.
Another monument opened around this same time was the Martin Luther King Jr. memorial. Like the 911 memorial there was a lot of controversies, due to race. This time it affected the design process. There was some protest over the statue being made by an Asian and not an African American. even though it was African Americans who chose the artist, based on artistic merit.