Cape Town - The City of Cape Town should commission a forensic investigation into the cost of its 2010 soccer stadium, Cosatu Western Cape said on Wednesday."The fact that the price nearly doubled after the initial budget should have raised a red flag to the most inexperienced accountant," the union federation said in a statement.The increase had to raise serious questions about the suitability of the city's procurement practices."These questions can only be answered by an independent forensic audit that investigates all the parties involved," it said."The City of Cape Town should initiate this forensic audit, given the location of the stadium and the involvement of the mayor's office."Last week the Cape Regional Chamber of Commerce and Industry said the city would be compelled to take legal action against construction firms should there be proof of collusion in the stadium bid.The Competition Commission has said all five of South Africa's major companies were involved in anti-competitive behaviour. One of the projects the commission investigated was the Cape Town stadium, where costs ballooned from R2.9bn to R4.3bn.