Police are appealing for witnesses after a stained glass window at Braintree Town Hall was smashed.
Five youths were seen to force open a side door of the building in Fairfield Road at 3.15pm on May 21. No entry was gained.
One then threw a tennis ball at the window, smashing the 12ft high window.
A member of staff disturbed the group, who ran off.
The boy who threw the ball was described as white, aged between 15 and 17, and dressed in black clothing including a hooded top with the hood up.
Another of the group was described as a white boy aged between about 15 and 17, with short ginger hair and wearing glasses.
Witnesses or anyone with information are asked to contact Braintree police station on 101.