(1) Lawyer's practice certificate;
(2) Power of attorney;
(3) Letter of introduction or legal aid from the law firm. The letter of introduction or official letter of legal aid issued by a law firm shall be stamped with the seal of the unit, indicating the basic information of the client, the name of the lawyer undertaking legal affairs, the ID number, the practice license number and other information, and indicating the information content to be collected and obtained and the reasons or cases related to undertaking legal affairs.
The relevant units can copy and save the above certificates and materials. In addition to the above documents and materials, the relevant units shall not attach other conditions or refuse lawyers to exercise their right to investigate and collect evidence according to law for other reasons, except as otherwise provided by laws and regulations. Lawyers and law firms that obtain information shall bear the legal responsibilities of collecting, obtaining information, keeping and using the obtained information according to law.
Legal basis: Criminal Procedure Law of People's Republic of China (PRC).
Article 124 Investigators may question witnesses on the spot or at the unit, residence or place proposed by the witness. When necessary, the witness may be notified to testify in a people's procuratorate or a public security organ. When questioning witnesses on the spot, they should show their work certificates, and when questioning witnesses at their units, residences or places proposed by witnesses, they should show their certificates from people's procuratorates or public security organs.
The questioning of witnesses should be conducted separately.
Article 125 When questioning a witness, the witness shall be informed of the legal responsibilities that he shall truthfully provide evidence and testimony and intentionally commit perjury or conceal criminal evidence.
Article 126 The provisions of Article 122 of this Law shall also apply to the questioning of witnesses.
Article 127 The provisions of this section shall apply to the interrogation of victims.