Hamas has beefed up security around its leaders following the arrest of several members of the Islamist movement on suspicion of "collaboration" with Israel and with Arab and Western intelligence agencies, sources close to Hamas told The Jerusalem Post on Thursday. The sources said some of the suspects were also involved in a plot to assassinate senior representatives of Islamic Jihad and Hamas in the Gaza Strip, including Ismail Haniyeh, Mahmoud Zahar and Said Siam. The sources refused to name the Arab and Western intelligence agencies that are said to have recruited the alleged collaborators. The turmoil in Hamas reached its peak this week when a number of top Hamas officials were summoned for questioning by the movement's security forces on suspicion of involvement in the alleged plot. Among those interrogated was Sami Abu Zuhri, a prominent spokesman for Hamas, the sources told the Post. However, Abu Zuhri denied Thursday that he had been questioned and accused the rival Fatah faction of spreading "lies" against Hamas leaders. The Hamas security forces have also interrogated Muhammad Abdel Al (Abu Abir), a senior commander of the Popular Resistance Committees, an alliance of radical armed groups closely associated with Hamas. The sources said Abdel Al was questioned following the assassination of one of his colleagues, Mubarak al-Hasanat, and a top Islamic Jihad commander, Majed al-Harazeen. The two, who were responsible for firing rockets at Israel, were killed by the IDF. Abdel Al has also denied the charges. The arrests have left the top brass of Hamas in disarray, the sources said, noting that tensions between top members of the movement reached a boiling point late Wednesday with the assassination of Hazem Muhammad Khalil. Khalil, who was a senior member of Izzadin Kassam, Hamas's armed wing, was gunned down by another top member of the group, Ra'fat Salman. The assassination is believed to have taken place in the context of a power struggle between rival factions inside Hamas. Earlier this week, Hamas officials expressed fear that Israel had succeeded in infiltrating Izzadin Kassam. They said at least two of the suspected "collaborators," brothers Ahmed and Yakoub Nassar, were senior members of Izzadin Kassam. Until his arrest, Yakoub Nassar served as an aide and bodyguard to Yusef Zahar, a prominent Hamas figure and head of the movement's paramilitary Executive Force. Ahmed and Yakoub Nassar reportedly confessed to helping Israel, together with other Hamas members, in the assassination of at least six top Hamas and Islamic Jihad operatives in the Gaza Strip over the past two years. Ahmed Nassar reportedly told interrogators he had been recruited by Israel's security forces more than three years ago convinced his brother to work with him earlier this year. Yakoub Nassar is said to have confessed to tipping off Israel about the movements of Majed al-Harazeen, the commander of Islamic Jihad's armed wing, who was killed in an IAF air strike last week. The confession came after a picture of Harazeen was found in Yakoub Nassar's possession. Another member of the Nassar family, Akram, served as a personal bodyguard to slain Hamas leader Abdel Aziz Rantisi. Palestinian security sources said they had not ruled out the possibility that the bodyguard was also working for Israel at the time.