JERUSALEM — Hamas closed its only civilian border crossing with Israel on Sunday, and Israeli troops were on high alert as tensions between the two enemies continued to rise, two days after a senior Hamas operative was mysteriously shot dead at point-blank range in the parking garage of his home.

Hamas has accused Israel of being behind the killing of Mazen Fuqaha, 38, a senior commander in the militant Islamist movement’s military wing. He spent nine years in an Israeli jail for his part in planning numerous suicide bombings that killed multiple Israeli civilians during the second Intifada in the early 2000s.

Fuqaha was eventually one of more than 1,000 Palestinian prisoners released in 2011 in exchange for Israeli soldier Gilad Shalit. Upon his release, Fuqaha was barred from returning to his childhood village in the West Bank and expelled to Gaza. From there he has managed Hamas’s military operations in the West Bank.

The tensions Sunday as a result of his death were the highest since 2014, when Israel and Hamas fought an intense 50-day war resulting in the deaths of about 74 Israelis and more than 2,100 Palestinians.

This is the first time Hamas has closed the Erez crossing into Israel, a checkpoint most frequently used by Gazans seeking health care in Israeli hospitals. Gazans go to hospitals in Israel and the West Bank. Erez is also used by aid workers and foreign journalists wanting to enter the strip. Hamas, which Israel and the United States consider a terrorist organization, also set up checkpoints across the strip looking for those who might have collaborated with Israel to kill Fuqaha.

“We say it clearly that the crime was planned and conducted by the Zionist enemy. And the enemy will be responsible of the crime consequences,” Hamas’s military wing, the Izz ad-Din al-Qassam martyrs brigade, said in a statement. They threatened that Israel would pay the price for killing one of its senior commanders.

While leaders in Gaza declared with certainty that Israel was behind the attack, a close-up assassination such as that carried out Friday — Gaza’s Health Ministry said Fuqaha had been shot at close range with a pistol fitted with a silencer — has not happened in the coastal enclave since Israel withdrew from the territory in 2005.

Fuqaha’s wife, Nahed Asida, said in an interview with Al Jazeera that the family had just returned from a day out. Her husband went to park the car in their basement garage and when he did not return for more than half an hour she thought he’d gone to see some neighbors. She learned of her husband’s death only when a friend came to tell her.

“I couldn’t believe it at all. It was a shock,” she said. “He received death threats all the time since his release in 2011, but he never paid any attention to it.”

While no one has yet claimed responsibility for the attack, Israeli media reported Sunday that the assassination is to similar in style to the killing of Hamas drone engineer Mohammed al-Zawari on Dec. 15, 2016, outside his house in Tunisia. That operation was attributed to the Mossad.

Additionally, Fuqaha’s father told Palestinian media that officers from Israel’s Security Agency had visited him several times, threatening that if his son did not halt attacks on Israel, they would get hold of him. During one such visit, the elder Fuqaha put the agents on the phone with his son.

“He was listed by the Israeli security bodies and his name was mentioned many times before but what is surprising is the way the assassination was conducted. It is the first time that Israel has operated in such way in the Gaza Strip,” said Ibrahim Ad-Madhoun, a columnist at Hamas affiliated newspaper Al-Resalah.

Until now, Israel has used missiles and jets to kill Hamas leaders from the air, he said.

Ad-Madhoun said what might have raised Hamas’s suspicions that Israel was behind the attack is that Fuqaha is not well known in Gaza but is responsible for Hamas’s activities in the West Bank. That would make him a target for Israel.

Although Hamas has yet to retaliate for the killing, Israel is bracing for a reprisal. Troops and communities in southern Israel were put on alert throughout the weekend.

Former director of military intelligence Amos Yadlin said in a public forum Saturday that the assassination could quickly spiral into renewed clashes between Israel and Hamas.

“Hamas could decide that Fuqaha was assassinated by Israel and retaliate, and then we will retaliate to the retaliation and we could very quickly in another clash very quickly,” he said.