A senior Hamas leader was shot dead near his home in the Gaza Strip on Friday, the group said, blaming Israel for the killing.
An Israeli military spokeswoman declined to comment on the incident in the Hamas-run Palestinian coastal enclave.
Mazen Fuqaha, a leader from the occupied West Bank whom Israel released in a prisoner swap in 2011 and exiled to the Gaza Strip, was shot several times, said Hamas police.
Another senior Hamas official, Izzat El-Reshiq, said the killers used silencers.
Thousands of people were expected to turn out for Fuqaha's funeral on Saturday.
"Hamas and its (military wing) hold (Israel) and its collaborators responsible for this despicable crime... (Israel) knows that the blood of fighters is not spilled in vain and Hamas will know how to act," the group said in a statement.
Khalil al-Haya, Hamas's deputy chief in the Gaza Strip said only Israel would have had something to gain from the death.
Fuqaha, 38, was one of more than 1,000 Palestinian prisoners that Israel released in exchange for soldier Gilad Shalit whom Gaza militants had held captive in the coastal enclave after abducting him in a cross-border raid in 2006.
Israel jailed Fuqaha in 2003 for planning attacks against Israelis and sentenced him to nine life terms. Israeli media said that after his release while in exile in Gaza, he continued to plan attacks by West Bank militants.
Cross-border violence between Gaza militants and Israel has largely died down since a 2014 war in which militants launched thousands of rockets into Israel.
According to Gaza health officials, more than 2,100 Palestinians, mostly civilians, were killed in 50 days of fighting. Israel put its dead at 67 soldiers and six civilians.
Militants from small Jihadist Salafi groups have continued to fire an occasional rocket into Israel from Gaza despite Hamas's efforts to rein them in but Israel says it holds Hamas responsible and responds with air strikes and tank fire.
Hamas has recently said it is becoming impatient with Israel's bombing of its facilities and has hinted that it may end the current state of relative calm.