Scientists investigating the Yucatan crater (believed to be the impact site of the asteroid that killed the dinosaurs) now suggest that the impact happened too far back in time to have caused the extinctions.
A core taken from the middle of the crater suggests an impact 300,000 years before the geological layer marking the sudden extinction of the dinosaurs. New magnetic evidence also agrees that the asteroid could not have directly killed the dinosaurs.
This leads to the belief that the mass extinction must have come about in some other manner, although it is still fully possible that the asteroid impact in the Yucatan was a contributing factor. Source