Teniendo en cuenta las funciones y responsabilidades que debe tener tal especialista, yo lo traduciría como
especialista en [investigación de] circunstancias atenuantes de la pena de muerte.
Es bastante largo, pero me parece que da más indicios sobre lo que hace esa persona.



In describing the scope of relevant mitigation evidence the 2008 Supplementary Guidelines explain:
(…)

With respect to the mitigation specialist, that person:
must be able to identify, locate and interview relevant persons in a culturally competent manner that produces confidential, relevant and reliable information. They must be skilled interviewers who can recognize and elicit information about mental health signs and symptoms, both prodromal and acute, that may manifest over the client’s lifetime. They must be able to establish rapport with witnesses, the client, the client’s family and significant others that will be sufficient to overcome barriers those individuals may have against the disclosure of sensitive information and to assist the client with the emotional impact of such disclosures. They must have the ability to advise counsel on appropriate mental health and other expert assistance.28

“Mitigation specialists must possess the knowledge and skills to obtain all relevant records pertaining to the client and others. They must understand the various methods and mechanisms for requesting records and obtaining the necessary waivers and releases, and the commitment to pursue all means of obtaining records.”29 In 1998, the Judicial Conference of the United States likewise recognized the significance of mitigation specialists in federal capital cases, even encouraging Federal Defender Offices to have such specialists on staff as permanent salaried employees.30 The accompanying commentary described mitigation specialists as “part of the existing ‘standard of care’ in a federal death penalty case.”31
(https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1160329)


Cases are used to illustrate how to identify, evaluate and present mitigation evidence in assessing issues of culpability in the mitigation of punishment in death penalty cases. It also exposes mitigation professionals to recent developments in the social sciences with implications for assessing issues of practical rationality, diminished volition, unfortunate forms of socialization, criminal propensities, socio-cultural deprivation, and gang involvement. These topics are linked with legal and philosophical conceptions of moral culpability that offer mitigation professionals new ways of thinking about both proximal and remote forms of mitigation. These socially oriented lenses, used in examining these concepts and legal issues, offer alternative ways of thinking about issues of capacity, choice and character in assessing diminished forms of moral culpability. The book concludes with recommendations for future research and other strategies for promoting the improvement of practice in the field of capital mitigation.
(https://www.amazon.com/Death-Penalty-Mitigation-Specialists-...


https://www.google.co.ve/search?newwindow=1&ei=3cr2WqvHKsLH5...