USC's (birth certificates) - need help finding my grandfathers.

My family is from Biała Podlaska in eastern Poland from what we're all told. All US family documents say Warsaw, though. I sent letters to the Urząd Stanu Cywilnego of Warsaw, Biała Podlaska and Radzyń Podlaskie (another nearby large area) specifically for my grandfather's birth certificate. All three found nothing.

Is this most probably because:
1. It was destroyed in WWII (he was born in 1918)
2. It was never reported (things were obviously a little different back then I can tell from my research)
3. I did something incorrectly

I don't know the extent of #1 or #2, so I don't know what's most probable. Should I try something else?

Since you know where he was born, it would be worthwhile checking the parish churches for baptisms etc., Let me know if you need any help with Polish.

There was a Kruszewski who taught mathematics at Lilford Technical School in UK

The post above is correct you want to find the proper roman catholic parish that your family attended in Warsaw. You may want to try this link to contact someone and they may be able to give you the correct parish for your relatives birthplace. catholic.li.pl/organisation.html. Do you have exact dates of your grandfathers birth.