Georgetown - Police in Malaysia have arrested 20 people in a widening investigation into the murder of at least 18 Myanmar nationals in the northern state of Penang since January.Twelve Myanmar migrants have confessed to their role in nine of the murders, which have seen bodies dumped - many with their throats slashed - across the state, Penang police chief Abdul Rahim Hanafi told reporters on Thursday.Police now believe two unrelated groups are behind the murders, Abdul Rahim said. All of those arrested are from one group.The widening net follows the reported discovery of a "slaughterhouse" last week, where some victims were believed to have been killed and dismembered.Media and community groups have speculated that the murders have been perpetrated by Muslims from Myanmar, likely including stateless Rohingya, as payback for religious violence in their home country since 2012.Myanmar community organisations have told Reuters the majority of the victims were Buddhists.The police have not revealed the ethnic identity of the victims and have denied that the killing were due to ethnic or religious conflict."There are no local suspects. This is a feud between Myanmar people. It is not a religious conflict. These are only vengeful murders that were brought over here from Myanmar," Abdul Rahim said, without elaborating.Police on Thursday released the pictures of two men from Myanmar, identified as Mohammad Yahyar Khan and Lokman, who are being sought for questioning.Buddhist community leaders have accused the police of being slow to act and refusing to acknowledge a religious or ethnic motive for the murders."It's just talk when the police say 'no Rohingya are involved,'" Aye Tun Maung, a leader of Buddhist community group the Arakan Refugee Relief Committee, told Reuters."It's Rohingya killing the Rakhine. Some of the dead are also Bamar and Shan," he said, referring to majority Buddhist ethnic groups.More than 60 000 migrants from Myanmar live in Penang's industrial heartland, many of them illegal arrivals, asylum seekers and refugees who are employed in dangerous jobs in factories and construction sites.Penang is also a destination for stateless Rohingya, more than 100 000 of whom have fled Myanmar by boat since outbreaks of religious violence in 2012.