He was arrested in December 2016[1] and accused of being involved in the Kurdistan Communities Union (KCK) which Turkey sees as an extension of the Kurdistan Workers' Party (PKK).[7] He was released in March 2017 pending trial.[1] In a separate case, he was sentenced to one year and six months imprisonment for spreading propaganda for the PKK on terror related charges in February 2019.[8] The KCK case was treated as part of the KCK mail trail and as of September 2019, the case was still ongoing.[9] On the 17 March 2021, the State Prosecutor before the Court of Cassation of Turkey Bekir Şahin filed a lawsuit before the Constitutional Court demanding for Önlü and 686 other HDP politicians a five-year ban to engage in politics.[10] The lawsuit was filed jointly with a request for a closure of the HDP due to the parties alleged organizational links with the PKK.[11][10]