Egyptian intelligence is not certain, but their theory is sensible. A couple of days before the Grad missiles from Sinai, there was a Grad missile fired at Ashqelon, another very rare event. To imagine these firings of Grad missiles as just coincidental seems to stretch the imagination. It makes sense that they were coordinated, in which case the organization that fired from Gaza is also tied to the firing from Sinai. Note that I have excluded the possibility of a copycat attack from Sinai. This was because it takes a good deal of preparation, especially to look for a safe place, in order to fire from Sinai. A copycat would not have had the time.
U.S. defense contractor accused of copying secret military documents (Reuters)
from the article: Netanyahu: Hamas responsible for rockets on Eilat; we will retaliate