The culprit is Sachi Makise. Prior to murder, she put wool coat on Tamba and in the elevator she used her necklace as a ground wire to connect electricity from floor to Tamba's pacemaker thus, destroy it and end his life. Makise confessed that she did that because Tamba rejected her proposal. Saito revealed that Tamba rejected her because he often told him that Makise is so perfect that she will find someone better than him. This put Makise in tears.
Afterward Genta fully declared that he's indeed the best leader of Detective Boys which is objectioned by the rest.