Police in Nigeria’s southeast Imo state are investigating after gunmen allegedly from the separatist group the Indigenous People of Biafara (IPOB), shot and killed a traditional ruler and three others. The group has previously denied responsibility for a series of attacks in the region that authorities blame on the rebels. The killings Monday came as a court in the capital dismissed terrorism charges against the separatist group’s leader. Imo state police said in a statement that gunmen disguised as locals in distress invaded the palace in the Oguta local government area around noon Monday, shooting and killing the monarch. The gunmen also killed two of his aides and a local vigilante member near the palace. On Tuesday, Nigerian President Muhammadu Buhari condemned the murder of the traditional ruler and called on police to investigate. Imo state police spokesperson Michael Abattam told VOA by phone they're already heeding the president's call. "We're investigating already and we have clues," Abattam said. Abattam could not provide further details. IPOB has repeatedly denied involvement in a surge of violent attacks in southeastern Nigeria over the past two years. Between May and October of last year, authorities said at least 175 people, including military, police and local civilians were killed in attacks in the region. Imo state is a strong base of support for the Biafran separatist movement, which began decades ago. The movement is now led by 54-year-old Nnamdi Kanu, who is facing trial for acts of terrorism and treason against the Nigerian state. Kanu’s legal counsel, Ifeanyi Ejiofo, says authorities are trying hard to implicate the separatist movement. He said it's a conspiracy by the government of the day and security agents to blackmail IPOB before the international community. He noted that IPOB has issued several publications distancing itself from those committing crimes. These are serious offenses, these people should be hunted and treated like criminals, he said, but they're not IPOB members, they're not Kanu’s followers. Experts warn violence in the southeast could increase around elections next year, after unidentified gunmen issued warnings on social media for people not to vote. Meanwhile, on Monday, an Abuja high court adjourned Kanu's terrorism trial indefinitely. Last month, a three-judge appeals court panel in the capital held that Nigerian authorities breached local and international treaties to unlawfully arrest and detain Kanu and annulled terrorism charges against the separatist leader. Nigerian authorities are challenging the ruling and have yet to free the separatist.