An explosive device hidden in a heavily guarded compound where Ismail Haniyeh was known to be living in Iran killed him, according to a New York Times investigation, according to seven Middle Eastern officials, including two Iranians, and an American official.

According to five Middle Eastern officials, the bomb was hidden about two months ago in the guesthouse.

The Iranian Revolutionary Guards are supervising the protection of the compound where Haniyeh was assassinated, which is part of a large complex in an upscale neighborhood north of Tehran.

Haniyeh was participating in the inauguration ceremony of the Iranian president, and when he entered the room where the decision was made, the planted bomb was detonated, leading to his martyrdom, according to officials who spoke to the American newspaper.

The explosion also killed one of Haniyeh's bodyguards.

The explosion shook the building, shattered some windows and caused an exterior wall to partially collapse, according to the Iranian officials, two members of the Revolutionary Guard who were briefed on the incident.

The American newspaper says: "Haniyeh had stayed at the guest house several times when visiting Tehran," according to Middle Eastern officials.

"All the officials spoke on condition of anonymity to share sensitive details about the assassination," she said.

Iranian and Hamas officials said Wednesday that Israel was responsible for the assassination, an assessment also made by several U.S. officials who asked not to be identified.

Israel has not publicly acknowledged responsibility for the assassination, but Israeli intelligence officials briefed the United States and other Western governments on the details of the operation in the immediate aftermath, according to the five Middle Eastern officials.

On Wednesday, Secretary of State Antony Blinken said the United States had no prior knowledge of the assassination plot.

In the hours after the assassination, speculation immediately focused on the possibility that Israel had killed Haniyeh with a missile strike, perhaps launched from a drone or aircraft, similar to how Israel fired a missile at a military base in Isfahan in April.

The missile theory raised questions about how Israel was able to evade Iranian air defense systems again to carry out such a daring airstrike on the capital.

“But it turns out,” the New York Times says, “the assassins had managed to exploit a different kind of gap in Iran’s defenses: a hole in the security of a supposedly heavily guarded compound that allowed a bomb to be planted and kept hidden for weeks before it was finally activated.”

Three Iranian officials say such a breach would have been a catastrophic failure of intelligence and security for Iran and a huge embarrassment for the Guards, which use the compound for lodging, secret meetings and housing prominent guests such as Haniyeh.

The American newspaper added: "It is not clear how the bomb was hidden in the guest house."

It quotes what it called Middle Eastern officials as saying that planning the assassination took months and required intensive surveillance of the compound.

The two Iranian officials who described the nature of the assassination said they did not know how or when the explosives were planted in the room.

The New York Times indicates that Israel decided to carry out the assassination outside Qatar, where Haniyeh and other senior members of Hamas's political leadership live.

The newspaper says: “The deadly explosion early Wednesday shattered windows and caused part of the compound’s wall to collapse, images showed and Iranian officials said. It appeared to have caused limited damage outside the building itself, as a missile likely would have done.

“At about 2 a.m. local time, the device exploded,” she continued, according to Middle Eastern officials, including Iranians.

Panicked building employees ran to find the source of the loud noise, which led them to the room where Haniyeh was staying with his bodyguard, officials said.

There is a medical team in the complex that rushed to the room immediately after the explosion.

The team announced that Haniyeh was killed instantly. The team tried to revive the bodyguard, but he was also killed.

Two Iranian officials said the leader of Palestinian Islamic Jihad, Ziad al-Nakhalah, was staying in the room next door. His room was not significantly damaged.

Khalil al-Hayya, Hamas's deputy leader in the Gaza Strip who was also in Tehran, arrived at the scene and saw Ismail Haniyeh's body, according to the five Middle Eastern officials.

Among those immediately notified, the three Iranian officials said, was Gen. Esmail Qaani, the commander of the Quds Force, the Revolutionary Guards’ foreign arm, which works closely with Iran’s allies in the region, including Hamas and Hezbollah.

The commander-in-chief of the Quds Force woke Khamenei up and informed him of the assassination.

Four hours after the explosion, the Revolutionary Guards issued a statement that Haniyeh had been martyred.

By 7 a.m., Khamenei had summoned members of Iran’s Supreme National Security Council to his compound for an emergency meeting, during which he issued an order to strike Israel in retaliation, according to the three Iranian officials.

Tehran was already under tight security for the inauguration of Iran's new president, Masoud Pezeshkian, as senior government officials, military leaders and dignitaries from 86 countries gathered at the parliament in central Tehran for the ceremony.

In Iran, the method of the assassination has been the subject of rumor and controversy.

Tasnim News Agency, the Guards' media outlet, reported that witnesses said a missile-like object hit the window of Haniyeh's room and exploded.

But the two Iranian officials, the two Guard members who were briefed on the attack, confirmed that the explosion occurred inside Haniyeh's room, and said that an initial investigation showed that the explosives had been placed there some time ago.

They described the attack’s precision and sophistication as similar in tactics to the remote-controlled, artificial intelligence robot weapon that Israel used to assassinate top Iranian nuclear scientist Mohsen Fakhrizadeh in 2020.