Six crude bombs went off near this election rally by Indian opposition contender Narendra Modi in Patna city on Sunday.
Another exploded at a railway station.
At least five people were killed and more than 80 wounded.
Doctors at the hospital where many of the victims are being treated say some are in a critical condition.
On Monday police were searching the area for clues.
They said a suspect they'd arrested had admitted the aim was to cause a stampede among the crowd of thousands.
Four other people have been detained for questioning.
Such explosions at large political rallies are unusual.
It wasn't immediately clear who was behind the bombings and there was no claim of responsibility.
It was the first such incident since Modi launched his campaign to oust the Congress party-led coalition that's governed India for a decade.