IT WAS to be London’s answer to the High Line, New York’s ribbon of park in the sky. The north end swathed in wisteria and summer snowflake, primrose and willow covering the south, the Garden Bridge would have connected Temple Underground station with the South Bank of the Thames. But on August 14th the charity that had been overseeing the construction of the bridge cancelled the project, following the withdrawal of financial guarantees by Sadiq Khan, London’s mayor. Serious questions about the management of the aborted project are now being asked.

In recent years London has put up plenty of jazzy new constructions. The London Eye, which cost £110m ($143m) in today’s prices, and the Millennium Footbridge, which cost £30m, have drawn in tourists and money. Boosters for the Garden Bridge saw it playing a similar role. Sceptics, however, questioned the sense of building a new crossing to a busy part of the South Bank barely 300 metres from Waterloo Bridge, while east London remained short of river crossings.

The bridge’s financial foundations were hardly solid, either. Its business plan relied on rosy assumptions about the revenue that the project would generate. It hoped to hire out the bridge for £60,000 an evening, when people can use venues such as the National Portrait Gallery for less than half that amount.

In 2013 the projected capital cost was £60m-100m. In 2013-14 the Department for Transport and Transport for London (TfL), the city’s road and rail authority, committed £30m of funding each. Yet since then expenses have spiralled. The latest estimate put the capital cost at over £200m. The task of raising the extra cash fell to the Garden Bridge Trust, a charity, which tried to persuade wealthy folk to make big donations.

But donors were less forthcoming than hoped. With the project facing a shortfall of £70m, Mr Khan could have been asked to finance a bigger proportion of the bridge’s construction. Under an agreement with the previous mayor, Boris Johnson, the mayor’s office also faced ultimate liability for the estimated £3m in annual running costs. With costs rising and other sources of revenue drying up, Mr Khan refused to provide such guarantees.

The Garden Bridge Trust and TfL have already spent £37m of public money on the project and even now may have to spend another £9m, despite the fact that construction never got close to starting. All infrastructure projects have upfront costs. Yet the trust has allocated around a quarter of its total capital budget in this way, which is unusually high, says Christian Wolmar, a transport expert.

Last October Margaret Hodge, a Labour MP, was commissioned by Mr Khan to investigate the tendering process. Her report suggests that Heatherwick Studio, which won a contract to design the bridge, had little experience in the art. Its initial proposal looked pricey beside others. According to Ms Hodge’s report, Heatherwick Studio will earn £2.7m from the abandoned project. Meanwhile in July 2013 Arup, an engineering firm, was employed as a consultant. After the company had submitted its bid, TfL asked the firm to revise it. No other bidder received such treatment. TfL’s own review of the tender said that it “did not follow TfL procurement policy and procedure”. Arup, whose work on the bridge included selecting “the warm-coloured alloy for its attractive finish”, has earned at least £8.4m from the affair.

It is hard to account for the remaining £35m or so of public funds. The Garden Bridge Trust’s published accounts are not detailed and, like most charities, it is exempt from freedom-of-information laws. Lawyers’ fees have undoubtedly eaten up a chunk. The trust says that money was spent on “procuring and placing orders for materials”, including steel. This spending is surprising, since no agreement was finalised to build on what was to be the bridge’s landing site on the South Bank. One architect suspects that sitting in a Thameside warehouse are millions of pounds’ worth of prefabricated parts for the bridge.

Mr Khan might have refused to give further support sooner than he did. Yet his predecessor, Mr Johnson, has bigger questions to answer. Documents relating to the tendering process have been destroyed, according to Ms Hodge. Richard de Cani and Isabel Dedring, who worked on the project for TfL and as the deputy mayor for transport respectively, have since joined Arup (Ms Hodge found “no connection between Arup’s contract with TfL for the Garden Bridge and their recruitment”). Mr Johnson, who is now foreign secretary, refused to appear before the Hodge review, saying that it “would not be the best use of my time”. Taxpayers who will see nearly £50m of their money spent on not building a bridge may disagree.