New research casts doubt on the theory that a single asteroid wiped out the dinosaurs.
Scientists have often pointed to a crater in Mexico as the asteroid's impact point.
But Princeton University researchers say the impact that caused the crater occurred 300,000 years before the dinosaurs were wiped out 65 million years ago.
A report appears in this week's online edition of Proceedings of the National Academy of Sciences.
At least one scientist doubts the group's findings. Richard Norris of the Scripps Institution of Oceanography says the Princeton researchers were working with incorrect site data.