Hamas officers in Gaza
Hamas officers in Gaza Arutz Sheva: Flash 90

Sources in Gaza reported Thursday Hamas evacuated the headquarters of its 'security forces'  fearing Israeli retaliation for the three-stage terror attack attack that left at least 6 Israelis dead and 33 wounded -- including 2 children aged 4 and 7.

It was also reported Israel closed the Kerem Shalom border crossing without further notice. The central cargo channel was shut immediately after the terror attack.
Hamas voiced concerns that Israel "is trying to export the crisis that has befallen it, and transfer it to Gaza."
Defense Minister Ehud Barak hinted harsh retaliation will follow, saying while visiting the south that "Gaza is a source of terror, and we will take full-force action against them."
"There will be a price tag to this event," Barak said. "Gaza will be severely hit. It is clear that the plan (for the attack) came from terror organizations in Gaza…The IDF will soon prepare for a response." 
Prime Minister Benjamin Netanyahu conferred with Barak earlier over security issues. The Prime Minister's Office said Israel's response to the attack would be swift.
On Wednesday it was reported Hamas was harboring Al-Qaeda terrorists active in recent attacks in Sinai and had refused repeated requests by Egyptian authorities to surrender them.  
It is unknown if Ramzi Mahmoud Al Mowafi, slain Al-Qaeda leader Osama Bin Laden's personal physician and bomb maker -- recently spotted in Gaza -- is among the terrorists Hamas is protectig.
The terrorists in today's attack are believed to have crossed the Israel-Egypt border into Israel from Sinai, where Egyptian security forces are conducting counter-terrorism operations.