According to sources, last June Libya has £182m on deposit with the bank, representing money it had collected for oil deposits. By September that figure had risen to £870m.
The Mail quotes unnamed sources suggesting the Americans want to know whether officials at the bank actually indulged in bribes to ensure that the deposits ended up at HSBC.
It's worth stressing once more that at this stage nobody has confirmed officially that an investigation exists, and all of the usual principles of "innocent until proven guilty" would apply here just as they would anywhere else.
Banks are currently not obliged to disclose details of dealings with nation states. It's ironic timing for the allegations to emerge, however, as today the Bribery Act comes into force in the UK today.