-54 their ability to obtain any type of documents, to organize high quality expert queries, to compel witnesses both inside and outside detention facility to testify and to issue obligatory orders or recommendations. But often they are unable to gain the trust of the detainees and in particular the trust of the victims, which is of a key importance for the effective monitoring, especially in the case of persons in a situation of vulnerability in detention. The situation with the NGO monitoring is often the opposite. They are usually able to gain the trust and to establish rapport with the victims, but their monitoring powers are often limited. In some cases, they are not even allowed to talk with some prisoners, e.g. with those detained on remand. NPMs are in a good position to benefit from the strengths of both official and the non-governmental monitoring approaches. Their mandate is established by law and the OPCAT provides for a comprehensive list of powers, which member states must confer to the NPMs. In Europe they are usually part of or national human rights institutions themselves, which, according to the Paris Principles, are supposed to enjoy independence from any governmental authority. As such, they should not be viewed by the detainees as involved in or complicit to their criminal prosecution, or as agents of governmental control and thus untrustworthy. Article 21 of the OPCAT provides for immunity from sanctions against any person or organization for having communicated to the NPM any information, whereas Article 22 provides for a duty of engagement of national authorities with NPM recommendations. 3.3.1. Choice of groups and identification of standards The focus of monitoring the rights of groups of detainees in a situation of vulnerability has to be based on a variety of criteria, both universally valid and country-specific. Certain groups are probably vulnerable in every system of criminal detention, because of the prevailing negative stereotypes towards them in society, their specific needs which the prison system cannot meet, as well as their physical weakness and/ or disability. These are factors of general vulnerability. The European systems of criminal detention differ in their ability to take account of and accommodate different vulnerabilities. Thus, some are better equipped