The BBC said its investigation found that the Glazer family's debts are 400 million pounds ($570 million) greater than previously known.The extra debt comes after the Glazers borrowed extensively against their shopping mall business in the United States, according to the finding being broadcast Monday night on the Panorama program.The extent of the debt could fuel a further revolt by a group of United fans who oppose the Glazers' ownership.The Glazers, who also own the Tampa Bay Buccaneers of the NFL, took over United in 2005 in a leveraged buyout worth 790 million pounds ($1.4 billion).The BBC said it saw mortgage documents showing that the Glazers have borrowed $570 million against shopping malls owned by their company Allied Corporation. That's in addition to $1 billion in debts tied to Manchester United and $95 million to the Buccaneers.The Glazers have said they have assets totaling 2 billion pounds ($2.9 billion)."I don't think anybody can be satisfied with how Manchester United are being run," Dave Whelan, chairman of Premier League club Wigan, told Panorama. "They have got somewhere in the order of a three-quarters of a billion pounds worth of debt. That has got to be eliminated and eliminated quickly." The research for the BBC program was carried out by Andy Green, a London financial analyst who is part of a campaign by disgruntled United fans to oust their Glazers.Thousands of United fans have attended matches at Old Trafford this season adorned in green and gold colors instead of the club's usual red. The homage to United's original 19th century colors is a protest against the Glazers.In March, United chief executive David Gill said the club's debts were easily covered by the income."In an ideal world people would like to not have a mortgage on their house but that doesn't mean they don't enjoy the benefits of living in that house and can't afford that house," he said.The Glazers have said they will not sell the club.British media have widely speculated that the family rejected a 1.5 billion pound ($2.2 billion) offer from a Qatari group last year.Forbes magazine recently estimated that United is worth $1.8 billion — making it the most valuable football team in its rankings for the sixth straight year.