A small fire was started in a council building after a vandal threw a piece of rubble through a ground floor window.
Offices of Daventry District Council in Lodge Road, Daventry, were targeted sometime between 8.20pm and 8.35pm on Saturday, March 7.
The fire caused damage to a window blind and the windowsill.
Police are treating the incident as arson.
A Daventry District Council spokesman said: “The fire was picked up by the smoke detectors and we understand that a passerby also called the fire service, who attended to put it out. Thankfully the fire was contained quickly and the damage to the building was minimal.”
Witnesses, or anyone with any information about this incident, can call Northamptonshire Police on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111 or use the anonymous online form at www.crimestoppers-uk.org.