There are some pretty sick and twisted people out there, and some come in the form of those who stuff tennis balls with explosives as a means of targeting dogs.

Homemade "tennis ball bombs" began popping up in Washington state, after which the Kent Police Department issued a warning to dog owners, KUTV reports.

The bombs contain explosive material found inside of fireworks, which is added to a tennis ball with a wick created at the top of the ball. Police say some of the bombs are very effective, whereas others don't even work. Still, you never know which ones just might hurt your dog.