There are initial reports that mortars that hit near the Kerem Shalom crossing came from Egypt.
In the hours before Operation Pillar of Defense, there were four rockets fired from the Sinai towards Israel.
I can imagine the Islamists in the Sinai itching to join in the shooting, especially since they know Israel isn't going to attack Egypt. At least one major Sinai jihadist group openly claims to cooperate with Hamas when firing rockets.
Egypt, publicly a staunch defender of Hamas but privately far more cautious, will need to make clear very soon how it intends to deal with such actions. Morsi will probably string this out as long as possible, claiming plausible deniability to the West while tacitly supporting the groups by dragging his feet to stop them.
As far as I can tell, Egyptian security did little or nothing after the last round of fire on Israel.
Even if this report doesn't pan out, there is a good chance that Sinai groups will be firing towards Jewish communities.
The target of Kerem Shalom is interesting. It is the crossing through which Gazans get most of their food and other supplies via Israel. The crossing was closed on Sunday and Monday because of the rocket fire, reopened Tuesday and Wednesday and then closed again since then.
(By the way, Israel keeps the Erez crossing open for diplomats and reporters. A number of diplomats, trying to flee Gaza on Thursday, were stopped from leaving - by Hamas. They were detained for several hours. Too bad none of them will talk about it. I received this information from an Israeli military source.)