In Monday's briefing, police said the teacher, 53, is accused of obstructing school business by leaking exam questions and correct answers five times between June 2017 and July 2018.

The daughters face the same charge, they said.

Police said they have found substantial evidence that suggests the twins had been involved in the alleged cheating.

They allegedly hid the answers in memo apps and created their own keywords to memorize them. On their actual exam papers, faint traces of some answers were found written beside each question, police said.

Forensic evidence shows the daughters exchanged details via their mobile phones about answers to an English exam days before they took the test.

Police believe that the father teacher accessed the exam questions while he was on night duty, of which he never left any record.

Police launched a probe following an inquiry by the Seoul education board, which concluded there were enough grounds for a possible crime. The education office's own inquiry came after some parents raised doubts on the Internet about the girls' oddly sudden jump in the ranks.

The teacher has denied all the allegations. The daughters are said to have also denied any wrongdoing. (Yonhap)