Two cars suffered extensive damage when they were forced open and lit fireworks were placed inside.
The incidents involved vehicles parked on Lumley Avenue and Essex Gardens, in South Shields, last week.
Police say this was an exceptionally dangerous act which could have caused serious injury to either the offenders or passers by.
A witness saw two youths committing the offence on Essex Gardens and police are appealing for information about them.
They are particularly keen to speak to adults in the area who are aware of local youths who may be in possession of fireworks at this time of year.