Why wasn’t there any attempt to solve the crime involving Bod’s family?

In The Graveyard Book, the main character, Bod, starts out as a baby whose parents were murdered. The baby wanders into a graveyard, where two ghosts, Mr. and Mrs. Owens, decide to adopt him.
It turns out that Bod’s physical parents were murdered by an organization called the Jacks of All Trades. They gain magical power from killing. However, the reason why they killed Bod’s parents was due to an ancient prophecy. According to a white-haired man who was a Jack, the prophesy stated that


There would be a child born that would walk the borderland between the living and the dead. That if this child grew to adulthood it would mean the end of our order and all we stand for.


The prophecy referred to Bod, who grew up in a graveyard.
This is why the deadliest Jack of them all, Jack Frost, was sent to kill Bod when he was a baby. But Jack Frost failed, and Bod was protected by a graveyard full of spirits, including his ghost foster parents, the Owenses.
In chapter 7, Bod’s friend Scarlet does some research into Bod's parents' murder. She has trouble finding anything about it, and there was no investigation.
In the same chapter, one of the Jacks, who is referred to as “the blonde man,” says:


Can’t leave any witnesses, not to something like this.


The Jacks plan on killing Bod, Scarlet, and anyone else who knows about the Jacks, in order to protect themselves and the secret of their weakness.
There was no investigation because the Jacks covered up the murder.