The Muslim Brotherhood was founded in Egypt in the 1920s and is the source of pretty much every extremist Muslim group, including Hamas. The Muslim Brotherhood continues to be active in Egypt and is the most obvious alternative to the Army/Mubarak regime should that regime fall. Stratfor, citing a source inside Hamas, says members of that organization may be slipping into Egypt to aid the Brotherhood:
The Egyptian police are no longer patrolling the Rafah border crossing into Gaza. Hamas armed men are entering into Egypt and are closely collaborating with the MB. The MB has fully engaged itself in the demonstrations, and they are unsatisfied with the dismissal of the Cabinet. They are insisting on a new Cabinet that does not include members of the ruling National Democratic Party.
Security forces in plainclothes are engaged in destroying public property in order to give the impression that many protesters represent a public menace. The MB is meanwhile forming people’s committees to protect public property and also to coordinate demonstrators’ activities, including supplying them with food, beverages and first aid.
Stratfor cautions that Hamas has an interest in exaggerating its role and that the above report should be read with that in mind. Nevertheless, “there is a great deal of concern building in Israel and the United States in particular over the role of the MB in the demonstrations and whether a political opening will be made for the Islamist organization in Egypt.”