What Happened to My Housemartin Nests?

Something seems to have attacked my 2 housemartin nests in the night - they have been nesting here for a few years now, started with 1 nest and then made another, it was lovely having them and every year they have produced broods. But this year I recently found one nest in pieces on the ground, previously to this a baby martin dead on the ground some way away from the nest which I couldn't understand. Last night I heard some odd noises which sounded like they were outside but on a high level. This morning I found pieces of the second nest on the ground, and saw a big hole in the side of the nest. It looks like something has attacked the nests, they wouldn't just fall into pieces like this :( could it be an owl? The reason I ask is that last week my son noticed a large owl perched on the ridge of our conservatory roof, something I'd never seen before, and I wondered if the owl might have done it?  But wouldn't it have eaten the baby rather than drop it?  So upsetting and I am so sad for the martings and I can't get up there to protect them, so I wonder if they are gone for good now :(