Izz El-Deen Sheikh Khalil was not a front rank leader of Hamas. But he is believed to have been a senior figure in its military wing.
By Alan Johnston
BBC correspondent in Gaza
He was nearly 40 years old, which in Hamas's terms made him something of a veteran. He was drawn from that generation that set up the organisation.
Mr Khalil came from the Shajaiyeh district of Gaza City, a Hamas stronghold.
He was among a group of around 400 Palestinian militants who were deported to Lebanon in the early 1990s.
While many were later able to return to the occupied territories, Mr Khalil stayed away, apparently fearing that he would be rearrested.
He based himself in Damascus along with a number of senior Hamas figures in exile.
With the organisation coming under more and more pressure in the West Bank and Gaza, the Syrian capital became an increasingly important centre of activity.
After the Israelis killed the two leading Hamas men in Gaza, the movement's control and direction was seen as shifting decisively to its top men in Damascus.