Even more controversial is the suggestion that the Cretaceous extinction event was only part of a larger group of impactors that had been impacting Earth for billions of year of it early history.
Researchers base their new timeline on layers of material that fell back to Earth after being thrown into space. This material seems to suggest a bombardment period of between 3.5 and 1.7 billion years while the standard models of an heavy impact period of from 4.1 to 3.8 billion years.
The IO9 article goes on to suggest that these asteroids may have come from a section of the no long extinct part of the asteroid belt that existed between Mars and Jupiter
Here is the link to the IO9 article.