BEFORE initiating the criminal case: The staff personnel of the entities of operational activities, whose affiliation with an entity of operational activities shall be classified and coded, and adult persons who covertly co-operate with entities of operational activities, with whom the employees of the entities of operational activities have formed an agreement (verbally or in writing) with respect to clandestine co-operation, shall be the covert participants in operational activities. They can carry out the operational actions and imitate the model of the criminal action. The imitation of the criminal act model is the approved actions, formally having the features of the criminal act or other law violations, and which are carried out in order to safeguard state protected person’s rights and freedoms, property, and society and state safety against the criminal attempt. AFTER initiating the criminal case: Article 158 of the Criminal Code of the Republic of Lithuania provides a possibility to the officers of the pre-trial investigation, as the procedural measures of constraint, to conduct the investigation not revealing their identities. This procedural measure of constraint may be applied only when the pre-trial investigation is commenced and the pre-trial judge’s sanction is obtained, which is based on a prosecutor’s reasoned motion. It is also needed to have enough data that a person, who is under the investigation, has committed a criminal offence. The officers are granted the right to carry out the actions, which imitate the criminal act.