Hamas said on Thursday that Mossad agents carrying Bosnian passports were behind the assassination of Mohammed Zawahri, the Palestinian militant group's drone expert, in Tunisia last year.

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Mohammed Nazzal, a member of Hamas' political bureau, said during a press conference in Beirut that an investigation showed that, as part of the preparations for the assassination, two apartments were rented. He Zawahri was under close Mossad surveillance for four months prior to his killing, and that the Mossad agents were aided by other intelligence agencies. Nazzal added that Mossad agents carrying Bosnian passports were responsible for his death.

Following the assassination, Hamas formed an investigation panel headed by Nazzal. "We've reached clear conclusions about who was behind the assassination," he said.

>> With killing of Hamas' drone expert, long list of alleged Israeli assassinations grows

Open gallery view Members of the Ezzedine al-Qassam Brigades, hold a banner bearing a portrait of one of their leaders, Mohammed Zawahri, during a ceremony in his memory on December 18, 2016 in Gaza City. Credit: AFP PHOTO / MAHMUD HAMS

This is not the first time Hamas has accused Israel of being behind the assassination. Shortly after the assassination, Iz al-Din al-Qassam, Hamas' military wing, Said that Zawahri was developing drones for the group, accusing Israel of his killing.

According to reports, Zawahri was shot at close range with six bullets, three to his head, while in his car. The reports said that the assassin was highly professional and skilled, and didn't leave a trace.

Zawahri, 45, was a former Tunisair pilot. He served as head of a south Tunisia civil aviation organization, and trained youth to fly drones.

Reports in Tunisia and around the Arab World said following the assassination that Zawahri was an enthusiastic supporter of the Palestinian cause. United Arab Emeritus' newspaper Al-Khaleej claimed that Zawahri received threats over the past few years due to his involvement with the manufacture of drones. The newspaper quoted a source saying that foreign elements are responsible for the assassination, but did not single out any particular country or intelligence organization.

The Lebanese newspaper Al-Akhbar, which is identified with Lebanon's Hezbollah Shi'ite militia, reported that Zawahri had worked with the military wing of Hamas since the 1990s, as well as with Hezbollah.

Tunisian authorities pursued Zawahri in the early 90s at which point he moved to Syria. In 2011 he returned to Tunisia after the former regime was toppled.

Previous assassinations also attributed to the Mossad include senior Hezbollah operative Imad Mughniyeh, who was blown up in Damascus in 2008; Hassan al-Laqqis, head of Hezbollah’s technological apparatus, killed in Beirut in 2013; and Mahmoud al-Mabhouh, a senior member of Hamas’ weapons smuggling network, killed in a Dubai hotel in 2010. And last December, Samir Kuntar – the Druze terrorist who worked for Hezbollah after his release from Israeli prison – was killed in an airstrike in Syria.

Israel neither confirms nor denies any of its alleged overseas activities, but says it reserves the right to fight terror even beyond its borders.