Platteville police are investigating what is believed to be an explosive device that went off inside a mailbox of a house near Union Street and Perry Drive Friday.

The Platteville Post Office contacted police after a mail carrier noticed what appeared to be an explosive device inside a residential mailbox.

Police determined that the bomb had gone off inside the mailbox.

Police believe the bomb was a “bottle bomb,” consisting of caustic chemicals inside a plastic container. The explosion, a loud bang, occurs when the chemicals react with each other, according to police.

Since the chemicals are caustic, they can injure someone who comes into contact with them, police said.

Anyone who sees a device that appears to be a “bottle bomb” is asked to contact police and stay away from the area until emergency personnel arrive.

Anyone with information on the incident is asked to call police, 348-2313.