Madurai Police said that timers were used to trigger the explosions.
Two low intensity explosions took place inside two government buses parked outside Madurai city on Wednesday night
No injuries were reported as the buses were empty.
The Madurai Police Commissioner Shailesh Kumar Yadav said, "The drivers of both these buses were sleeping on the roof and none was injured."
"We've also found timer devices used in these. We are on the job," he said
Tuesday the 29th September was the death anniversary of a suspected militant Imam Ali who was shot dead by the police in Bangalore in 2002.
He was on the run after an armed gang secured him and his associate Hyder Ali from Madurai police.