Tamil Nadu government on Thursday ordered a CBI probe into the attack on the office of Tamil daily Dinakaran in Madurai, which left three persons dead a day earlier.

Announcing this in the state assembly, Chief Minister M Karunanidhi said that said since the issue concerned his family members, a probe by the state police will not be sufficient.

Hence, a CBI probe has been ordered, he said.

Two computer engineers and a security personnel of Maran-family owned Dinakaran were killed on Wednesday when supporters of Karunanidhi's elder son M K Azhagiri allegedly attacked the daily's office and set it afire, angered over an opinion poll favouring his brother Stalin.

The newspaper is owned by Karunanidhi's grand nephew Kalanidhi Maran, brother of Union Minister Dayanidhi Maran.

Meanwhile, four more persons, including the husband of city Mayor Thenmozhi, were arrested on Thursday in connection with the attack, police said.

With this, the total number of those arrested in connection with the Wednesday's attack has risen to six.

Gopinath, husband of Thenmozhi, Mari, Prem and Saravanan were arrested on Thursday while 'Tiger' Pandi and Badsha were taken into custody on Wednesday night in connection with the attack that left three persons dead.

The situation in the city was normal on Thursday morning, police said, adding that security had been provided at the daily's office. Besides, police had been posted in sensitive pockets of the southern district.

Mari, Prem, Saravanan, 'Tiger' Pandi and Badsha had been arrested for offences including murder. They along with Gopinath had also been charged with trespassing and rioting, police said.

Some councillors alleged to be involved in the protest are likely to be arrested on Thursday, they said.

Besides, around 20 persons had been detained in connection with the violence and arson at the daily's office.

Police said they had acted on the tip off that the office could be attacked and had posted a team led by a deputy superintendent of police for security, but they were helpless against the arsonists who were armed with petrol bombs, stones and clubs, said a top official.