The motorcyclist traveling from Nepalgunj to Kohalpur opened fire at the office and left, according to Bichitra Sah, in-charge of the office.
Police found 16 bullet shells of small machine gun from the site.
The Janatantrik Madhes Mukti Tigers took responsibility for the attack by making a phone call to a local newspaper.
Biru Chaudhary, who identified himself as military commander of the outfit, said the firing was a warning to the police office that has arrested supporters of the group.
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