Suspected fabrication of data

A reviewer expressed suspicion that data were manufactured. We wrote to the authors saying that our reviewer would like to see the original data. The author replied that this research was carried out in the USA. We then wrote back suggesting that his co-workers in the US would probably be delighted that this work was being submitted for publication and would happily send over the data but that without this we could no longer consider the paper. The line went dead. What should we do next?