Riefenstahl used camp inmates as extras for at least one movie. When she shot the movie "Tiefland", a move version of the opera taking place in the Spanish Pyrenees, she needed "souther" looking extras and made use of imprisoned Roma and Sinti from the camps of Maxglahn in Salzburg and Berlin Marzahn.
What she basically did was the same thing as some companies, i.e. "renting" camp inmates for forced labor from the SS. Most of the extras she used in this film were subsequently deported to Aushwitz and killed there.
While it is impossible to prove that she knew they were going to be killed subsequently, apparently she promised some of them to help them but then did not.
Another thing is that Riefenstahl was a propagandist for the Nazi regime and while that might not be strictly criminal it is certainly a moral problem. But the use of prisoner forced labor can and was classified as a crime of which Riefenstahl was guilty but never tried for.
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To add to /u/commiespaceinvader 's answer
The problem with punishing Leni Riefenstahl postwar for her activities under the Third Reich was that her cinematic oeuvre fit into a very nebulous zone. While her films were propaganda that gloried the state and its leader, they did not engage in open hate-mongering as exemplified by films such as Jud Süß or Die Rothchilds. Nor was the famous director a member of the NSDAP, even though she clearly benefited from the patronage of Hitler. As such, Riefenstahl tried to portray herself postwar as an apolitical artist more interested in the human form and was thus caught up in historical events outside of her control.
This stratagem worked to a certain extent in the immediate postwar years. The governing Allied powers set up denazification tribunals for prominent Germans and the Western Allies generally operated along a five-tier classification system (in sequential order- major offenders, offenders/incriminated, lesser offenders, fellow-travelers/followers, exonerated). Although this classification system seemed quite logical and fair, in practice it was quite chaotic and the results could vary quite widely over Germany. Riefenstahl experience with the denazification tribunals was somewhat atypical compared to her contemporaries because of her status as a prominent public figure in the former regime's cultural scene. There was intense pressure both within the resurgent German press and among foreign newspapers to have Hitler's alleged chief propagandist tried. Between 1948 and 1952, Riefenstahl went through four different denazification tribunals.
Riefenstahl's first tribunal went off without much of a hitch, which helped to ignite a degree of opprobrium in the press. The Hamburg newspaper Die Welt responded to her exoneration by publishing documents from the Reich Chamber of Culture which showed a much stronger engagement with the regime. Her second tribunal in Freiburg in 1949 was partly inspired by the German magazine Revue's expose of Riefenstahl that alleged that she had used Roma and Sinti from the Maxglahn assembly camps for her film Tiefland. Riefenstahl contended that the Bild's documents were forgeries and that Revue's charges were unfounded. Riefenstahl claimed in this tribunal and her defamation suits against Revue that she delegated casting the exotic extras to her assistant director and production manager. Riefenstahl testified in her second tribunal that during this selection process that "I couldn't be there myself. I was still in the Dolomites, scouting visuals." Witnesses at the second tribunal were also rather hazy on the subject and could not verify if the woman they saw was Riefenstahl. The second tribunal also delivered a verdict of exonerated, which prompted enough outcry for a third tribunal, conducted in absentia which gave her the status of a fellow traveler.
Despite losing Riefenstahl's defamation civil suit, Revue continued with exposes of the director, alleging that she was a witness to massacres in Poland while acting as a war reporter whose footage would be incorporated into the film Feldzug in Polen. This prompted a fourth tribunal in Berlin which investigated her role in Poland and it came up that she had witnessed the massacre of Konskie, and Revue published a photograph of her at the shooting of twenty or more Polish civilians. Riefenstahl claimed that she was appalled by the sight and resigned as a war reporter immediately. Riefenstahl also used the tribunal to undermine the credibility of Revue by testifying the the magazine had offered her the photographs in exchange for payment. The fourth and final tribunal delivered the same verdict as the first two: exonerated.
Despite her legal exoneration, she was subject to continued press notoriety and her career definitely suffered. As a woman and artist whose patronage by Hitler had earned her more than a few enemies, she had very few protectors within the newly emergent West German establishment that would allow her to continue her career status quo antebellum, although the FRG government did countenance a secret deal with Transit-Film Distribution that allowed her to receive royalties from first Olympia and then Triumph des Willens (which was in violation of the German Federal High Court's ruling that the NSDAP held the rights to the latter, and thus royalties should not be paid). Riefenstahl would adamantly hew to her story that she was not responsible for the casting at Maxglahn and Marzahn and that the Roma and Sinti extras all enjoyed the experience and survived the war. Nina Gladitz's 1982 documentary Zeit des Schweigens und der Dunkelheit contradicted this narrative and found surviving extras from Tiefland that asserted she was there at Maxglahn and one of the extras told Riefenstahl that they were to be gassed at Auschwitz after the film. Riefenstahl sued Gladitz to prevent the documentary from reairing but the German courts ruled in favor of Gladitz with the exception that she cut out the sections on the director knowing about gassings because it was hearsay that could be interpreted as slanderous.
The question of how much guilt Riefenstahl bore for her activities is a somewhat thorny one. Gladitz's documentary, coupled with Riefenstahl's rather hands-on directing approach, strongly suggests that she perjured herself in her various tribunals. But even in such a case, her use of slave labor on this film paled before the wider employment of slave labor by German industrial conglomerates like Daimler or Krupp. Unlike the German director, these industrial magnates often had extensive defense networks postwar that worked to sabotage and minimize any wide-scale denazification and break-up of their large fortunes. Riefenstahl suffered under the double burden of being publicly associated with the Third Reich, but also lacking any real marketable skills or allies that would allow her to slide back into the postwar order as if the war had not happened.
Sources
Rother, Rainer. Leni Riefenstahl The Seduction of Genius. London: Continuum, 2002.
Trimborn, Jürgen. Leni Riefenstahl: a life. New York: Faber and Faber, 2007.
To add to the other good responses here, Susan Sontag wrote a review of a book of photographs of Sudanese people Riefenstahl released in the 1970s. In it, Sontag makes an excellent case that Riefenstahl retained a fascist aesthetic to the end, suggesting that she was not merely associated with the Nazis, nor opportunistically used Hitler's patronage as a way to get her work made, but rather truly believed in a Nazi racial hierarchy. In that sense, then, she remains beyond rehabilitation, regardless of her own protestations to the contrary.
This is cultural "crime" rather than actual, I suppose, but having read it I feel little doubt that Riefenstahl was a fascist to the end.