An explosive device hidden in a heavily guarded complex where Ismail Haniyeh was known to stay in Iran was what killed him, according to a Times investigation.

Ismail Haniyeh, a top leader of Hamas, was assassinated on Wednesday by an explosive device covertly smuggled into the Tehran guesthouse where he was staying, according to seven Middle Eastern officials, including two Iranians, and an American official.

The bomb had been hidden approximately two months ago in the guesthouse, according to five of the Middle Eastern officials. The guesthouse is run and protected by the Islamic Revolutionary Guards Corps and is part of a large compound, known as Neshat, in an upscale neighborhood of northern Tehran.

Mr. Haniyeh was in Iran’s capital for the presidential inauguration. The bomb was detonated remotely, the five officials said, once it was confirmed that he was inside his room at the guesthouse. The blast also killed a bodyguard.