Two hijacked planes hit the World Trade Center on 11 Sep 2001

Since the 11 September 2001 attacks on New York's World Trade Center there have been many disagreements over the design of the redevelopment of Ground Zero. Work finally began on the Freedom Tower this week.

Here we track the key developments over the last four years:

11 September: Two passenger planes seized by al-Qaeda hijackers crash into the World Trade Center, bringing down the twin towers.

11 October: Clean-up operation is ongoing with 200,000 tonnes cleared to this date.

12 December: Fires burning in the debris are finally extinguished.

13 December: A public viewing platform is opened.

People look onto the site from a viewing platform

11 March: Vertical beams of light are switched on at Ground Zero. The two beams mark six months since attacks.

30 May: Ceremony marks end of clean-up operation.

16 July:The Lower Manhattan Development Corporation (LMDC) releases six initial design concepts.

26 February: Polish-born US architect Daniel Libeskind's proposal is selected, consisting of a centrepiece Freedom Tower and a sunken memorial garden.

The tower is to be 1,776ft (540m) high, echoing the date of North America's declaration of independence.

Responsibility for the site is divided between New York and New Jersey governors, who own the land through the Port Authority, and Larry Silverstein, the leaseholder with the right to redevelop the office space.

16 July: Larry Silverstein demands changes on the grounds that there are not enough shops and offices in the original proposal. Daniel Libeskind agrees to give fellow architect David Childs a lead role in developing the design for the Freedom Tower.

23 November: The commuter railway station re-opens at Ground Zero.

19 December: New design for the tower released, including an unoccupied section at the top and windmills - which could generate 20% of the building's energy.

4 July: A granite block is laid as a cornerstone of Freedom Tower.

14 July: Mr Libeskind files papers to take Mr Silverstein to court, seeking $850,000 he says he is owed in unpaid fees.

The Freedom Tower has been redesigned more than once

4 May: New York Governor George Pataki says security problems mean the tower must be redesigned. Police fear it is not sturdy enough to withstand a terrorist truck bomb.

18 May: New York billionaire Donald Trump attacks the Freedom Tower design as "disgusting". He backs a plan by engineer Ken Gardner, which looks similar to the old Twin Towers - only one storey higher.

29 June: A redesigned Freedom Tower is unveiled boasting safety improvements and more closely resembling the character of the fallen towers.

Negotiations between Port Authority officials and Mr Silverstein continue throughout the year over who will build on Ground Zero, how much rent will be paid, and how to divide money paid to Mr Silverstein.

15 March: Port Authority officials pull out of negotiations saying Mr Silverstein had put his interests before those of the public. Negotiations continue soon after.

26 April: A deal is agreed between Mr Silverstein and the Port Authority. Construction work gets under way.

Under the deal, Mr Silverstein will hand over the Freedom Tower but remain landlord of three other planned towers.

A memorial and transit hub will open in 2009. The towers are scheduled for completion in 2012.