Carlo D'Este briefly recounts the Biscari incidents in his biography of Patton, A GENIUS FOR WAR, at pp. 509-510. According to the version in D'Este's book, there were two separate incidents, with the result that 73 Italian POWs were massacred by a sergeant and a captain in the US 45th Infantry Division. D'Este mentions nothing about the massacre of German prisoners. I believe the date was 14 July 1943. According to D'Este General Bradley ordered the two men court-martialed. Their counsel raised the so-called "Nuremburg defense" later asserted by German defendants, namely that they acted on the orders of higher authority. In this case, the "higher authority" was Patton, who on 27 June had delivered a "pep talk" to the men of the 45th ID in two shifts. D'Este does not have a transcript of Patton's speech, and I have been unable to locate it elsewhere. Whatever he said, there were those in the 45th who evidently interpreted it as an exhortation to take no prisoners. D'Este does not discuss the ultimate fate of the US sergeant and captain, although he does say that the Nuremburg Defense "was rejected".
Again according to D'Este, as a result of the incident Patton became the target of an official investigation. An officer of the inspector general's office visited him in England shortly before the Normandy invasion. The inspector general's report seems to have exonerated Patton.
Interestingly enough, I received what might be considered confirmation of the incident earlier this year. I received an email from a woman in Illinois who had read something I had written about Operation HUSKY. Her father, who had served in the 45th ID had recently died. She wrote to say that her father had told her for years that something terrible had happened on the day in question, but would never tell her what it was. She asked whether I knew anything about it, and I gave her the information from D'Este's book. The lady's father had been taken prisoner in Italy and privately pubished a short book on his experiences, a copy of which she graciously provided to me. In his book, her father specifically mentions the date in question, saying that "something terrible happended that day; I have never spoken about it, and never will." He didn't, either, going to his grave without telling anyone.
All the best.