The first salvage workers began sifting through the rubble within five hours of the collapse of the World Trade Centre.

What began as a search and rescue operation has grown into a mammoth clean-up effort involving the emergency services, the New York authorities and contractors including Amec and Bovis Lend Lease. More than 5400 people are still missing in the remains.

There are now thousands of workers working around the clock, and by Wednesday about 50,000 tonnes of rubble had been cleared. More than 350,000 tonnes remain and it is estimated it will take up to a year to clear the site – particularly since it is being treated as a crime scene, with the FBI sifting all the debris cleared for evidence.

The workers are using trucks, excavators and cranes to lift and transport the twisted steel girders and concrete that are the remains of the World Trade Centre.

New York's department of design and construction, which is co-ordinating the rescue effort, has divided the site into quadrants.

Amec and Bovis are overseeing work in the south-west quadrant, which takes in the remains of the south tower.

Turner Construction and Plaza Construction are managing the eastern section. This includes three buildings that have totally or partially collapsed.

Tishman Construction is responsible for the area that includes the northern tower, and Tully Construction has the remaining southern quadrant.

A subcontractor with one of the biggest presences on site is demolition firm Season Contracting. It is working around the collapsed south tower and has brought about 30 pieces of equipment to the site.