Tamil Nadu CB-CID, probing the case, had arrested four persons yesterday.

Sheik Mohammed Umar Shah alias Raju Was arrested for harbouring the four accused and keeping the bomb in his residence cum workshop and Jaleel (18) for carrying the bomb on a motorcycle driven by Kaleel to the building in the early hours of Sunday, CB-CID DIG, K Muthukaruppan told newsmen here on Friday.

Muthukaruppan said this was a ``clearly pre-planned criminal conspiracy'', hatched two months ago targeting police establishments here, Tiruchirapally and Chennai.

Investigations revealed that the four bombs, similar to those used in serial blasts in the city last year, kept in three places, were manufactured at Madurai and police have clues about the persons, who had manufactured, stored and distributed them tothe destinations, Muthukaruppan said.

However, he declined to divulge the details for fear of hampering further investigation.

Giving details of the operation, Muthukaruppan said that Kaleel Rehman, the main accused, had collected the bomb from Madurai and delivered it to Abdul Nazir at Attupalam, at the city outskirts on May 28. The zinc box bomb was then taken to the workshop of Raju in Nanjundapuram on a scooter. Raju had given shelter to other five accused and kept the bomb for a whole day at his workshop, in a sprawling one acre land, he said.