A German parliamentary committee on has begun hearings related to a child porn investigation involving a lawmaker, which has led to a major crisis in Chancellor Angela Merkel's two-month-old coalition. Several key players involved in the affair are set to be questioned by the parliamentary interior committee on Wednesday, German media reported. Among them are Thomas Oppermann, parliamentary group leader of Merkel's ruling partner SPD, SPD chief Sigmar Gabriel, foreign minister Frank-Walter Steinmeier and Jorg Ziercke, head of Germany's Federal Criminal Police Office, according to reports. A number of questions are expected to be cleared up during Wednesday's all-day meeting. A political uproar broke out last week, when former interior minister Hans-Peter Friedrich resigned on Friday amid accusations he leaked confidential information that a SPD lawmaker was on the list of child-porn purchasers to Sigmar Gabriel back in October last year. Authorities reportedly found the name of Sebastian Edathy on a list of customers of an online shop selling child pornography. Investigators searched Edathy's apartment and offices on Monday last week, but only found two computers with broken hard drives, media reports said. Edathy resigned from his seat in German parliament for "health reasons" days before the search action. On Thursday, Thomas Oppermann told media the SPD leadership knew about the accusations against Edathy in October. He also confirmed that Friedrich spoke to Gabriel about the case. Prosecutors said they would look into whether there's a case to answer relating to the betrayal of state secrets. Members of the opposition in parliament began to call for Friedrich's resignation. Friedrich told a press conference on Friday that he remained convinced he had acted appropriately when informing Gabriel, whose SPD were in coalition talks with Merkel's CDU and its sister party CSU following Germany's September elections, but the pressure surrounding the issue had grown too much for him to stay in office. The CSU politician said his intention with the tip-off was to make sure Edathy would not receive a post in Merkel's new cabinet, but did not know he may be implicated in a scandal. Prosecutors have complained that leaking the information could have compromised the case against Edathy, saying he appeared to have advanced knowledge of the investigation. Suspicion is mounting that SPD members may have warned their fellow party member about the investigation, possibly allowing him to remove evidence and delete illegal data from his computers. But Gabriel insisted that neither Steinmeier nor Oppermann had spoken to Edathy about the case. Friedrich's resignation has led to a rift between Merkel's coalition partners. CSU leaders have expressed frustration that the only high-level politician implicated in the scandal is a member of the Bavarian party. CSU head Horst Seehofer accused the SPD and especially Oppermann of "breach of trust" and called on SPD members to clarify their actions concerning the Edathy case, reports said. Merkel met with leaders of the two parties on Tuesday evening. She said she would seek to "restore trust" between her two coalition partners, adding that she expected questioning of witnesses on Wednesday to shed more light on the leak. Police are now investigating a growing list of possible crimes: whether Edathy's purchases were illegal, was his laptop stolen, who intercepted an official letter about the inquiry and whether Friedrich breached official secrecy.