Returning the rent deposit

[Bryan Laufenberg]

Recently purchased a triplex with tenants already in the property, thus I inherited their security deposits. One tenant had to leave but had been in the property for 4 years. Their security deposit was $450, they want their security deposit back but there are so many repairs needed and I have no idea if the damages were from them or not. They did smoke in the unit and I am wondering if I should give them back the deposit or keep it. 

Totally new to this so any advice would help.