Club staff called police to their training ground after a dead animal and a threatening sign, most probably left by Eintracht Braunschweig fans, were discovered on Friday
The two teams face one another on Sunday at the Eintracht Stadion, with some 3,300 policemen estimated to be in attendance to quell any hint of fan trouble.
And H96's stars have been delivered a chilling warning, finding the sheep next to a sign reading: "Your time will come on Sunday, you filthy animals".
Police have confirmed the incident, releasing a statement reading: "Today, at 07:00, a H96-employee saw a lamb on the fence of the H96 training complex and called the police.
"A vet has picked up the animal and checked the cause of death. The sign is being examined for traces. We're investigating for violations of the Animal Welfare Act."
Hannover sporting director Dirk Dufner was perplexed by the incident, telling Bild: "This is a twisted act of some individuals. This is incomprehensible and perverse."
The first derby between the two sides this season was marred by violence, with four people being arrested and 14 police officers sustaining injuries.