What is te right thing to do with the money we found in the house we bought?

Approximately 6 years ago we purchased an older home from an elderly lady who's brother passed away(Michigan). The brother was never married and died at approx. 90 years old with sister inheriting house. She cleaned out everything she wanted and ransacked the house as rumor had it the older brother had hoarded money away. (heard she found $200,000 in freezer)Anyway, since then, the sister has deceased and we have been cleaning up the house for the last 6 years since we bought it. Now, we have found a considerable anmount of money in basement and only known relative is a 60 year old Attorney who lives (or use to) in Virginia somewhere. Morally we want to do the right thing & return or tell someone but legally is the money ours? If so & I think it would be, is it reported to IRS as capital gain, income of some sort or how do you deal with money that's found?