Cities in the United States remain on high alert before a special day of ceremonies to mark the 10th anniversary of the September 11th attacks. Yesterday relatives of victims gathered in a field in Pennsylvania to watch the former President George Bush and current Vice-President Joe Biden unveiled a memorial to those who died in one of the hijacked planes. Forty men and women died in fields in Pennsylvania when United flight-93 slammed into the ground. In New York silence was start of the day, a minutes long across the city of the exact moment that the north tower was struck ten years ago.Tight security was already in place across the nation as the anniversary approached, and police bolstered their presence in New York and Washington after the tip was received several days ago. Officials said the threat of a possible attack would not disrupt ceremonies scheduled for today which is the anniversary itself.