LONDON -- The American owners of Manchester United are $1.6 billion in debt, leaving their control of the club in doubt, the BBC reported Monday.

The BBC said its investigation found the Glazer family's debts are $570 million greater than previously known. Malcolm Glazer and his sons also own the Tampa Bay Buccaneers.

The Glazer family has been the subject of protests by a group of Manchester United fans who want it to sell the team. Andrew Yates/AFP/Getty Images

All the extra debt comes after the Glazers borrowed extensively against their shopping mall business in the United States. The extent of the debt could fuel a further revolt by a group of United fans who oppose the Glazers' ownership.

The Glazers took over United in 2005 in a leveraged buyout worth $1.4 billion.

The BBC said it saw mortgage documents showing the Glazers have borrowed $570 million against shopping malls owned by their company First Allied Corporation. That's in addition to $1 billion in debts tied to Manchester United and $95 million to the Buccaneers.

The Buccaneers released a statement Monday trying to reassure their fans that the team is not in financial trouble.

"Buccaneers fans should know that the Glazer family is as financially well-positioned as ever before. Companies they own generate revenues in excess of $800 million each year," Jonathan Grella, the team's director of communications, said in the statement.

"Sophisticated real estate experts know that the family's refinancing of their commercial real estate before the global meltdown has proven to be the wise move.

"While First Allied represents only a small portion of their asset portfolio, it continues to generate significant profits, enjoys over 90 percent occupancy, and has long-term non-recourse financing.