Tesco dampened the food and drug retail sector on Monday as news of a fresh investigation into the struggling supermarket chain weighed on investor confidence.Accounting watchdog, the Financial Reporting Council (FRC), said that it has launched an inquiry into the "preparation, approval and audit" of Tesco's historic financial statements by the company's auditor PricewaterhouseCoopers (PwC).The FRC said it is looking at the retailer's accounts dating back to February 2012.Tesco is already under investigation by the Serious Fraud Office in relation to its £263m overstatement of profits for the first half, which resulted in the suspension and subsequent departure of a number of senior executives.Tesco's shares were down 2.4% at 180.9p by 16:01 and have now lost of 46% of their value since the start of the year.Rival supermarket J Sainsbury was also down 0.1% in afternoon trade, while Wm Morrison was trading broadly flat.Online grocer Ocado however was bucking the trend, rising 3.5% after positive comments from Numis Securities which maintained its 'buy' stance on the stock.The broker said that despite a "tough year" for the wider industry, Ocado has made "steady progress" with decent sales growth and "resilient" profits. It said Ocado "offers significant upside""In our view, Ocado is uniquely positioned to benefit from the inexorable shift of grocery shopping online, leveraging its market-leading IP to support Ocado Retail and develop high returning partnerships with global grocery retailers looking for the best online grocery fulfilment solution," Numis said. Top performing sectors so far todayIndustrial Metals & Mining 1,423.14 +11.07%Health Care Equipment & Services 6,391.31 +1.51%Fixed Line Telecommunications 4,666.83 +1.38%Real Estate Investment Trusts 3,084.67 +1.21%Media 6,495.26 +1.09%Bottom performing sectors so far todayFood & Drug Retailers 2,712.89 -1.56%Oil Equipment, Services & Distribution 16,619.44 -1.55%Mining 13,897.95 -0.96%Forestry & Paper 11,353.01 -0.47%Industrial Engineering 8,801.91 -0.45%