Back from the drawing board: Freedom Tower, safer than before

The city was forced to send the plans back for revision after the police department raised concerns that it remained too vulnerable. The complaint was seen as another setback in the effort to rebuild on the site, a project that has been beset by political and bureaucratic squabbling.

Under the new plans, the tower will still reach the height of 1,776 feet, to symbolise the year of American independence, but it will now sit on a reinforced and bomb-resistant cubic base clad in luminous materials, probably combining stainless steel and titanium.

Crucially, the structure will now be set back 90 feet from West Street, rather than by the 25 feet of the original plans. Police said that plan invited car bombs.

"It is unique, yet it subtly recalls, in the sky, the tragedy that has happened here," said David Childs, the project's architect, said.