Federal investigators haven't named a cause of the derailment, but they and other officials familiar with the investigation said the probe was focused on a brokenrail joint found in the wreckage after the accident.
Officials from the National Transportation Safety Board have said they are examining the broken section of rail and the "witness mark" it left on one wheel of the first train.
They'll hear from Richard Brown, the chairman of Eurostar - who concluded in a government review, published this week, that rail franchising is 'not broken'.
The Rail Accident Investigation Branch (RAIB) said the broken sections were part of an auger drill that had "penetrated the tunnel lining before falling on to the track".