The incident took place last night at Maranga police station where Awdesh Mandal, the husband of JD(U) MLA Bima Bharti, was in the lock-up, he said.
The Nitish Kumar government today once again came under attack of the opposition over two incidents involving its MLAs.
Eyewitnesses said the escape took place when the legislator, along with Purnea MP Santosh Kushwaha, was in the police station and hundreds of supporters of Mandal surrounded the police station to help him flee.
Sub Divisional Police Officer of Banmakhi, Kundan Kumar, would probe the escape, the SSP said.
Criticising the state government in connection with the incident, senior BJP leader Sushil Kumar Modi asked whether the "attack on a police station by supporters of husband of a JD(U) MLA was not a case of free run to criminals".
Former chief minister and Hindustani Awam Morcha (HAM) leader Jitan Ram Manjhi assailed Kumar over the incident and said his "claim of tightening law and order is dishonesty.