Knowledgeable birders who have seen the photos are split between Long-tailed and Parasitic. I can see the reasoning for Long-tailed, apart from the white under primaries (should be dark in adult Long-tailed?).
Three people responded that they think Peregrine could be the culprit. Indeed, I've seen a Peregrine plucking a Scripps's Murrelet at the nearby Wilcox Property, so Peregrines do hunt over the sea. One person thought Great Horned Owl could be a possibility.
