Fortunately, the organization's web resources are intact--they have been attacked before--but businesses nearby have suffered, including that of THHP's director, Harry Mazal. Orac, who's worked with the project, has more details.
Harry is a great guy and a major force in the fight against Holocaust denial. Even though THHP's resources, library, and servers were unscathed, Harry's business has suffered a major loss, and other businesses in the building were collateral damage; Harry has paid a heavy price for his dedication. We have some suspicions about who might have done this, but no concrete evidence yet. That makes any speculation other than that the perpetrators are likely to be either people or a group opposed to THHP's message and mission not advisable at this stage.
Visit TTHP and show the arsonists that they have failed to destroy this worthy effort.