Hamas leader Ismail Haniyeh arrived in Iran to attend the inauguration of President Masoud Pezeshkian. AFP

A team of agents set up by the Iranian government has interrogated and detained more than two dozen people, including senior intelligence officers, senior military officials and staff at a guesthouse run by Iran's Islamic Revolutionary Guard Corps (IRGC) in Tehran, a source familiar with the matter said.

The source said Iranian investigators believe the hit team hired by Israeli intelligence (Mossad) is still in the country and will seek to capture them.

Earlier, on August 2, the British newspaper The Telegraph quoted anonymous sources in the IRGC as saying that Mossad had hired IRGC personnel to plant an explosive device to assassinate Mr. Haniyeh. The incident was said to have become "a humiliation for Iran and a major security breach" in the IRGC.

The assassination attempt on Mr Haniyeh was originally planned in May, during his visit to attend the funeral of Iranian President Ebrahim Raisi, but the operation was unsuccessful due to a crowd inside the building, sources told The Telegraph .

One of the sources within the IRGC told The Telegraph that the assassination of the Hamas political leader was “an insult to Iran and a major security breach” for the IRGC, adding that an internal investigation was currently underway within the organization.

Haniyeh was assassinated after attending the inauguration of newly elected Iranian President Masoud Pezeshkian on July 30. Hamas blamed Israel and the United States for Haniyeh's death and vowed to retaliate. Earlier this week, the New York Times reported that Haniyeh was assassinated by a bomb placed in his room at the guest house two months ago.