Madurai Police summoned Dravida Munnetra Kazhagam president M Karunanidhi on Saturday as part of their investigation into the murder of senior DMK leader T Kiruttinan.
Karunanidhi has been asked to produce one of his letters to his son M K Azhagiri, who has been arrested in connection with the murder.
According to police sources, the summons was issued on Friday night, after obtaining the necessary orders from the court.
It is said the letter is the same one that the DMK chief had released to newspersons in Chennai this week, to claim that there were no serious political differences between Azhagiri and Kiruttinan.
Karunanidhi claimed the police at the instance of the All India Anna Dravida Munnetra Kazhagam government was misusing Azhagiri's letters to him and Kiruttinan to rope in the former in the murder case.
"The AIADMK is also using the case to tarnish DMK's image," he said.