Prosecutors are state officials who lead and supervise pre-trial investigations and uphold charges laid against defendants during trials.
Most cases are usually investigated by police, prosecutors in these cases supervise activities of pre-trial investigation officers and are involved when decisions on certain coercive measures are to be taken. However, even in cases being investigated by the police, prosecutors can decide to take over the whole investigation or to perform some parts of it themselves.
Prosecutors in these cases can also order police officers to take specific steps in investigation, to search for specific evidence, to question specific witnesses, etc. You can also appeal every decision or inactivity of pre-trial investigation officers to prosecutor (and to appeal decisions of prosecutors to their superiors).
When you think that certain steps in the pre-trial investigation shall be taken, you can submit written petitions on these issues either to pre-trial investigation officer or to prosecutor directly.
The prosecutor is the one who takes decisions either to terminate pre-trial investigations or to forward cases to courts for trial. In the latter case the prosecutor leading pre-trial investigation writes a bill of indictment and formulates charges against the defendant. Afterwards prosecutor upholds the charges laid during the whole trial procedure. He or she can also appeal judgments of courts of first instances, and to apply for a review of the case by the Supreme Court of Lithuania.
Please note that every prosecutor performs their functions in the case because you have suffered a crime. However they do not have to abide to your decisions, i.e. in most cases they have to start criminal procedure even if you do not wish your crime to be investigated (except some specific crimes and criminal infringements), they do not have neither right nor duty to terminate criminal procedure if you wish it to be terminated, they can appeal every judgment even if you do not want any appeal to be made, etc.