A 15-year-old boy from Hull has been arrested by police investigating the theft of post.
Humberside Police today said that two sacks of post had been stolen from Royal Mail Delivery vans.
A spokesman said officers wanted to hear from anyone with information which might help with the investigation or the recovery of the stolen mail.
The incident has prompted the force to remind drivers to wary of further thefts of parcels.
The spokesman said: "If you are making deliveries remember, when you leave your vehicle to never leave the engine running, remove the keys and lock the doors, never leave the doors open and lock all doors when you leave the vehicle."
Anyone with information on the theft or suspicious activity around delivery vans is asked to call the 101 non-emergency number, quoting reference 16/68486/17.