This is some key points of what i would say, and you can re word it into a paragraph however you need :)
•Look for fingerprints. You can look for fingerprints on the paper and compare them to those on each of the pens, this is assuming the criminal was not wearing gloves.
•Dog fur. Search the houses for dog fur. This is pretty self explanatory, if a house has any sign of dog fur or anything, then that person must have been in close contact with the dog, and would likely have dog fur on their clothes or shoes or something.
•Handwriting. Have each suspect write a message on a piece of paper and see who’s is the closest to that of the note.
•DNA. This is a dog being kidnapped, not a murder so the criminal probably didn’t put as much thought into how to be certain their DNA is not getting everywhere, so look for fingerprints, hairs, shoe prints, and other tell tale signs.
If i think of any more i’ll respond to this comment, lmk if there’s anything else and good luck! :)