A border police counter-terrorism unit and a unit from the internal security forces, known as the Shin Bet, entered Ibn Sina hospital on the outskirts of the city's refugee camp early on Tuesday, CCTV footage of the aftermath of the operation showed.
The units made their way to a room on the third floor and shot all three men in the head using pistols fitted with silencers, in an attack that took less than 10 minutes from start to finish, Israeli media said.
Israel said the dead men were Mohammad Jalamana, a spokesperson for Hamas's military wing, Basel Ghazawi, of Islamic Jihad, and his brother Mohammed. All three were allegedly active in the umbrella force known as the Jenin Battalion, a newly formed group that has engaged Israeli forces in fierce fighting during raids in the lawless city over the past two years.
The Israeli authorities said the three men had been involved in planning an attack similar to the one launched by the Palestinian militant group Hamas on 7 October that killed 1,140 people and sparked the latest war in Gaza, which has killed 26,000 people in the strip, according to the Hamas-run health ministry, and led to a humanitarian crisis.
Hamas said one of the people killed was a member, while the Palestinian Islamic Jihad said the other two were part of their group. In a statement, Hamas called the killings "a vile crime that will not go without response". It said: "The resistance forces, who have sworn to fight the occupation until it is expelled, are not afraid of the assassination policy."
The Palestinian health ministry condemned the attack. Hospital officials said Ghawazi was being treated after being partially paralysed in an Israeli airstrike on a Jenin cemetery in November that killed four, including a 15-year-old boy. All those who died in that attack were named by Palestinian media as members of the Jenin Battalion.
A hospital spokesperson, Tawfiq al-Shobaki, said there was no exchange of fire during the raid but that the Israelis attacked doctors, nurses and hospital security. In security camera footage, one of the undercover agents can be seen searching a man against a wall with his hands above his head.
"What happened is a precedent," he said. "There was never an assassination inside a hospital. There were arrests and assaults but not an assassination."
The aftermath of the execution.
The Israel Defense Forces (IDF) have found underground tunnels in the vicinity of hospitals in Gaza, although although journalists have been unable to verify claims that medical buildings have been used as cover for major Hamas command and control centres.
Ibn Sina hospital, on the outskirts of Jenin's poverty-stricken and well-armed refugee camp, has been stormed or targeted several times by Israeli forces over the past year, including in a three-day operation last summer that at the time was the biggest IDF raid in the occupied West Bank for 20 years.
Since the war in Gaza started, the West Bank and the contested city of Jerusalem have also seen an uptick in violence: more than 380 Palestinians have been killed, mostly in violent protests and army raids, and 3,000 people arrested, according to Palestinian Authority statistics.
The fighting in Gaza is now entering its fifth month. Israeli forces are still battling Palestinian militants in Gaza City as Hamas regroups its forces in areas that were occupied by the IDF months ago.
Fighting and bombardment has continued to rock the coastal exclave, where Israel is now targeting the southern town of Khan Younis. Another 128 people were killed overnight, the health ministry in the Hamas-run territory said.
Talks aimed at brokering a second ceasefire, led by Qatar, Egypt and the US, involve proposals for a 30-day pause in fighting and the release of dozens of Israeli hostages still held in Gaza in return for Palestinians held in Israeli jails. Progress on a deal remains elusive, according to statements on Tuesday from Hamas and Israel.
Comment: The Daily Mail reported further:
Footage from the hospital rooms after the attack: