They didn't look at the Wagner's suspiciously right away but there was one thing that started it according to the prosecutor in court today.
Child custody.
The prosecutor said that the Wagner's said everything was fine with custody but then BCI was hearing from other people the exact opposite, that there was a custody dispute between Jake and Hanna. But that wasn't all. BCI found it suspicious that the same thing happened with George's child. That the Wagner's were the ones who wanted to raise him and so they did what they could to get him then kept the mother out of the picture.
They realized that the Wagner's wanted to do the same with Jake's daughter, same pattern basically. Get her and raise her. This started the alarm bells ringing and they then did the search of the Wagner properties and found the 22 shell casings matching the ones at the crime scenes.
The Wagner's had a storage unit and they found the receipt for the shoes, smaller than the sizes George and Jake wore. This is different from us hearing that the receipt was found in Jake's truck. Storage could have been at that car lot.