From an interview on NPR's Talk of the Nation (March 2004):

Magnus Ranstorp: ... I think if you ask any Hamas official, they would see themselves as part of a chapter of the Muslim Brotherhood, the Egyptian Muslim Brotherhood that was created in 1928. The Muslim Brotherhood was one of the first forces that fought Israel in the war of independence in '48, '49. Hamas--or shall we say that what Yassin sort of was responsible for creating--was primarily in the social sphere. He created the Al-Mujamma Al-Islami, the social sphere which was registered with Israel in 1978. It was primarily at a time in the late 1960s and the '70s that Hamas was expanding its social activity and trying to re-Islamicize society from the bottom up. In the 1980s, the movement became somewhat more militant in the sense--in '84 Yassin was charged and he was also imprisoned for some time for involvement or at least association with some terrorist activity in having weaponry, and he remained in prison for a majority of the time in the late 1980s.

Of course, Hamas really, as we know it today, came out of the first Palestinian uprising that began on the 9th of December, 1987. A few days later in early '88, Hamas actually adopted the name The Islamic Resistance and began to participate in the uprising against Israel. In 1988, it also--and '89--began to want to pursue a more violent strategy. And progressively it has escalated using different means of violence, beginning in 1989, 1990, with a war of knives; kidnapping Israeli soldiers, that led to the expulsion of 415 Hamas and Islamic Jihad activists to South Lebanon; and of course, coming into contact with Hezbollah in Lebanon, the Lebanese Shiite group that had been fighting the Israelis, adopted the suicide bombing and imported that into the Palestinian territory and began its first operation in terms of suicide bombings in April of 1994....

Rashid Khalidi: I see that its turn towards violence is very much a function of its competition for leadership in the Palestinian polity with the PLO, now the PA--that is to say, it has moved in this direction in order to outflank the Fatah-PLO-PA bloc, which it has succeeded in doing. The net effect of its actions--not only suicide bombings, all of its actions--and of this approach has been that it has gone from having less than 20 percent support in most polls to having close to double that right now.