US marks 9/11 amid shooting scareObama's tribute to 9/11 victims
US President Barack Obama has led ceremonies to mark eight years since 11 September 2001, the deadliest terrorist attack on US soil.
Speaking at the Pentagon, Mr Obama paid tribute to the victims and said time would not diminish the loss.
He vowed to "never falter" in defending the US and pursuing al-Qaeda.
But the event was overshadowed by mistaken reports of a shooting, sparked by a Coast Guard training exercise on Washington's Potomac River.
In the defence of our nation we will never waver, in pursuit of al-Qaeda we will never falter
President Barack Obama
The dark day
The four hijacks
How disaster unfolded
In pictures: 9/11 anniversary
CNN had reported that 10 rounds were fired by the Coast Guard as a vessel tried to go through a security zone, after intercepting radio messages.
The Coast Guard later said in a statement that it was an exercise, and that it would conduct a thorough review to establish how the incident could have been perceived as real.
Presidential spokesman Robert Gibbs said that to his knowledge the White House had not been notified of the exercise but defended the Coast Guard's decision to hold it.
See a satellite image of Washington DC
However, he criticised CNN for failing to check their reports.
Tributes
Remembrance services were held across the US for the victims of the 11 September 2001 attacks.
Adam Brookes, BBC News, Washington DC
The security establishment in Washington DC went into sudden overdrive on Friday morning after a major national news network began reporting a serious security incident on the Potomac river.
After a protracted period of confusion, US government agencies, including the Department of Homeland Security, the Pentagon and the FBI began saying that they had no reports of any shots fired.
Then, the US Coast Guard said that it had been conducting a training exercise on the Potomac. No shots had been fired, but a simulation might have involved the words "shots fired" being put over a radio network.
In the hair-trigger atmosphere of Washington DC on the anniversary of the 9/11 attacks, it is perhaps not a surprise that any security incident garners immediate attention, but there will no doubt be uncomfortable questions as to why the Coast Guard chose that particular moment to conduct an exercise.
Nearly 3,000 people died when the four planes crashed in New York, at the Pentagon and in a Pennsylvania field.
Speaking at the Pentagon on his first 9/11 anniversary as US president, Barack Obama said the years would not diminish the loss of that day.
"In the defence of our nation we will never waver," he said. "In pursuit of al-Qaeda we will never falter.
Accompanied by Defence Secretary Robert Gates, he met members of victims' families and laid a wreath.
Meanwhile, thousands gathered for ceremonies in a square near New York's Ground Zero.
Vice-President Joe Biden attended the New York ceremony, where planes crashed into the World Trade Center towers, causing them to collapse.
Volunteers who helped in the aftermath of the attacks joined family members in reading the more than 2,700 names of the victims.
http://news.bbc.co.uk/2/hi/americas/8251166.stm