WASHINGTON — The blast that killed the Islamic State’s leader during a Special Operations raid in northwestern Syria last week was most likely caused by a large bomb the terrorist rigged to destroy most of his third-floor residence, senior U.S. military officials said on Thursday.

The explosion was so powerful that military officials now suspect that a child found dead on the building’s second floor was killed by the blast’s concussive force, not in a firefight between the child’s parents and the commandos. The child had no visible injuries from gunshots or falling debris, the officials said.

The Pentagon has acknowledged seven deaths — four civilians and three Islamic State fighters — in the raid to capture or kill the leader, Abu Ibrahim al-Hashimi al-Qurayshi. But the military officials acknowledged on Thursday that more bodies might have been recovered from the rubble after the commandos had left the scene. Rescue workers have said women and children were among at least 13 killed during the assault.

New details about the pre-dawn assault are emerging a week after President Biden said he had ordered commandos to seize the ISIS leader, rather than bomb the entire three-story building, to minimize the risks to civilians. Pentagon officials have said that 10 people, including eight children, were safely evacuated. Defense Secretary Lloyd J. Austin III said the military would review whether the mission had harmed civilians.

The two-hour raid in the town of Atmeh near the Turkish border came days after the end of the largest U.S. combat involvement with the Islamic State since the jihadists’ so-called caliphate fell three years ago. American forces backed a Kurdish-led militia in northeastern Syria as it fought for more than a week to oust Islamic State fighters from a prison they had occupied in the city of Hasaka.

The battle for the prison killed hundreds of people and offered a bleak reminder that even after the collapse of the caliphate, and now the death of Mr. al-Qurayshi, the group’s ability to sow chaotic violence persists. Indeed, a United Nations counterterrorism report issued this week estimated that the Islamic State still retains 6,000 to 10,000 fighters across Iraq and Syria, “where it is forming cells and training operatives to launch attacks.”

Also this week, the State Department offered a reward of up to $10 million for information leading to the identification or location of Sanaullah Ghafari, the leader of Islamic State Khorasan, or ISIS-K, the group’s branch in Afghanistan. The terrorist group claimed responsibility for an attack at Kabul’s international airport on Aug. 26 that killed 13 U.S. service members and as many as 170 civilians during the American-led evacuation.

On Thursday, two senior U.S. military officials described the planning and execution of the raid to a small group of reporters on a teleconference. They spoke on the condition of anonymity to discuss operational matters.

The mission, led by Delta Force commandos, was set in motion last September with a tip that the ISIS leader was hiding out on the top floor of a house in northwestern Syria. Overseen by the military’s Central Command, the commandos rehearsed dozens of times, and Mr. Biden was briefed on an exercise involving a tabletop model of the building. The troops also practiced using a mock-up of the building that they would eventually raid.

By late December, the commandos were ready and Mr. Biden approved the mission. But bad weather in northwestern Syria and a desire to carry out the mission on a moonless night pushed the operation to Feb. 2.

The American assault in Atmeh, backed by Apache helicopter gunships and armed MQ-9 Reaper drones, resembled the raid in October 2019 in which Abu Bakr al-Baghdadi, the previous leader of the Islamic State, died when he detonated a suicide vest as U.S. forces raided a hide-out not far from where last week’s operation took place.

American officials alerted Israel, Turkey and Russia, which has troops based in northwestern Syria, shortly before the mission was underway to avoid any accidental contact, the officials said.

American officials have previously said Mr. al-Qurayshi, also known as Hajji Abdullah, lived with his wife and two children on the building’s third floor. He left the building only occasionally to bathe on the rooftop. He relied on a top lieutenant who lived on the building’s second floor and who, along with a network of couriers, carried out his orders to ISIS branches in Iraq, Syria and elsewhere in the world. A Syrian family with no apparent connection to the terrorist group was living on the first floor.

Shortly after the commandos arrived just after midnight, warnings shouted in Arabic over bullhorns urged occupants on the first floor — as well as anyone else — to evacuate. A man, a woman and four children fled the first floor.

At almost the same time, a huge explosion — much bigger than a suicide vest with 5 to 10 pounds of explosive, officials said on Thursday — ripped through the third floor. The blast was so powerful that bodies, including Mr. al-Qurayshi’s, were blown out the window.

Mr. Biden said last week that Mr. al-Qurayshi died when the terrorist exploded a bomb that killed him as well as members of his family. Military officials said on Thursday that they had no proof that Mr. al-Qurayshi detonated the bomb but thought so, given his position. The officials emphasized that the U.S. commandos did not attack the third floor or detonate any explosives, and caused none of the casualties.

After the blast, commandos stormed the building and engaged in a firefight with Mr. al-Qurayshi’s lieutenant and his wife, who were barricaded on the second floor with their children. Both died, as did one child, but four children were safely evacuated, U.S. officials said.

A 13-year-old boy who was among those evacuated from the first floor described his family’s terror at being taken from their home in the middle of the night.

The commandos had thrown his father to the ground and kicked him before picking him up and searching his body for weapons, the boy said, giving only his first name, Muhammad, for fear of retribution.

“I felt like I had reached my death and that there was no escape,” he told a reporter for The New York Times two days after the raid. “I figured when I saw them throw my father to the ground that they were going to kill him, to shoot him.”

His mother fled the house later and the commandos had torn off her head scarf and dragged her by her hair, he said.

After the operation, the Americans questioned the family about their upstairs neighbors and they replied that they had not known them well, he said.

Before the Americans left, they told the family, “We, here, killed the leader of ISIS,” the boy said.

The four children who were evacuated from the house after their parents were killed in the firefight on the second floor included two boys — a baby and a 2-year-old — and two girls, 3 and 12, he said.

The commandos left them with his family, the boy said, and they were taken the next morning by Hayat Tahrir al-Sham, an Islamist group formerly linked to Al Qaeda that controls the area.

The group has not said where it took the children.

Eric Schmitt reported from Washington, and Ben Hubbard from Beirut, Lebanon. Muhammad Haj Kadour contributed reporting from Atmeh, Syria.