The US has started to mark the 10th anniversary of the 9/11 attacks on New York, Washington and Pennsylvania.
Security is tight following warnings of a possible al-Qaeda attack.
The US embassy in Afghanistan has begun the ceremonies, with events due later in the sites where four hijacked planes struck, killing nearly 3,000 people.
An official memorial to those who died is to be unveiled at the site of the World Trade Center, whose twin towers were destroyed in the attacks.
Metal barriers have been erected on roads near the World Trade Center, while police in New York and Washington are stopping and searching large vehicles entering bridges and tunnels.
The CIA received a warning last week that al-Qaeda may have sent attackers, some of them possibly US citizens, to bomb one of the cities.
The warning was described by officials as "credible but unconfirmed".
President Barack Obama has said the US remains vigilant against terrorism.
As in previous anniversaries, the names of all the victims will be read out at the New York event.
There will be pauses for silence at the exact times when two airliners smashed into the World Trade Center's twin towers, the third was crashed into the Pentagon and the fourth was forced into a field in Shanksville, Pennsylvania, by passengers who fought with the hijackers.
BBC
Security is tight following warnings of a possible al-Qaeda attack.
The US embassy in Afghanistan has begun the ceremonies, with events due later in the sites where four hijacked planes struck, killing nearly 3,000 people.
An official memorial to those who died is to be unveiled at the site of the World Trade Center, whose twin towers were destroyed in the attacks.
Metal barriers have been erected on roads near the World Trade Center, while police in New York and Washington are stopping and searching large vehicles entering bridges and tunnels.
The CIA received a warning last week that al-Qaeda may have sent attackers, some of them possibly US citizens, to bomb one of the cities.
The warning was described by officials as "credible but unconfirmed".
President Barack Obama has said the US remains vigilant against terrorism.
As in previous anniversaries, the names of all the victims will be read out at the New York event.
There will be pauses for silence at the exact times when two airliners smashed into the World Trade Center's twin towers, the third was crashed into the Pentagon and the fourth was forced into a field in Shanksville, Pennsylvania, by passengers who fought with the hijackers.
BBC
