Go to this site: http://www.telefony.arteq.org/
. Click on PODLASKIE. Type in Mroz. In all of the area of this province, there is one person listed named Wojciech Mroz in the village of Lapy-Lynki.
Kaminski is a more common name, so you would want to know in what village he was born. Knowing that they were from the Lomza area is some help, but not much because Lomza is not only a city, but is also a county by the same name and there used to be a province with this name.
Since Mroz is not a common name in the area, I suggest writing to Wojciech to see if he is your cousin. He may be the last of the family.