Gaza City - Hamas' newly elected supreme leader on Thursday announced the arrest of the man believed responsible for the mysterious shooting death of one of his organisation's top military commanders in March, a breakthrough in a case that has embarrassed and shocked the Islamic militant group.Hamas hopes the arrest will shore up its standing with the public at a time when it has been weakened by the shooting, an economic crisis and rising tensions with the rival Palestinian Authority. Accusing Israel of orchestrating the assassination, Hamas declared victory and scheduled celebratory rallies across Gaza.Ismail Haniyeh, who was named the head of Hamas' political bureau last week, announced the arrest at a hastily scheduled news conference. He declined to identify the suspect but implied he was a Palestinian who had collaborated with Israel."All the evidence we have indicates that the perpetrator committed this crime based on orders from the Israeli occupation," Haniyeh said. More details will be released in the coming days, he added, noting that the suspect would face execution.The death of Mazen Faqha, a shadowy senior figure in Hamas' military wing, has shocked the militant group that has ruled Gaza with an iron fist for the past decade.Faqha, 38, was killed in the garage of his apartment building on March 24, shortly after he had dropped off his family. Hamas said the killer used a weapon with a silencer, allowing him to escape undetected before Faqha's body was discovered an hour later.At the time, the group quickly blamed Israel and imposed a lockdown on Gaza. It set up military-style checkpoints throughout the small, seaside territory, sealed Gaza's border with Israel and rounded up hundreds of people for questioning.'A strategic achievement'Israel has not responded to Hamas' accusations that it was behind the killing. Israel's Foreign Ministry said Thursday it would have no comment."What the enemy did was a painful strike in terms of strategy and security," Haniyeh said.He thanked the public for showing its "understanding" of the security measures and praised his intelligence services for cracking the case."What happened was a strategic achievement," he said. "The world will be surprised by the level of this mighty work."Human rights groups repeatedly have criticised Hamas for torturing people in its jails and for running a system of justice that does not meet international standards. In April, it executed three alleged "collaborators" with Israel in what was widely seen as a response to the Faqha shooting.But Thursday's high-profile announcement indicated the group is confident it has found Faqha's killer. Haniyeh stood next to Faqha's smiling wife, Nahed, as he delivered the news.Hamas organised victory celebrations in dozens of neighbourhoods across Gaza on Thursday night. In the southern city of Rafah, jubilant supporters handed out sweets.The group is desperate for a lift after a series of setbacks. Israel and Egypt have maintained a blockade on Gaza since the group seized control of the territory from forces loyal to Palestinian President Mahmoud Abbas. Israel says the blockade is needed to prevent Hamas, which seeks Israel's destruction, from smuggling in weapons, but the closure has hit Gaza's economy hard.The blockade was tightened after the 2013 military ouster of Egypt's Islamist President Mohammed Morsi, a close ally of Hamas. In 2014, Hamas lost hundreds of fighters in a war with Israel that also inflicted heavy damage on Gaza.The shooting of Faqha exposed flaws in Hamas' security procedures and its inability to find the assailant quickly caused further embarrassment. More recently, Abbas, who governs the Palestinian Authority in the West Bank, has cut financial support to Gaza, further squeezing Hamas.Faqha, originally from the West Bank, was serving nine life terms in Israeli prison for directing suicide bombing attacks before he was freed with more than 1 000 other Palestinian prisoners in exchange for one Israeli soldier in 2011. Under the release, Faqha was sent to Gaza.Haniyeh's appointment came shortly after Hamas' rulers unveiled a new, seemingly more pragmatic political program aimed at ending the group's international isolation.Hamas is trying to rebrand itself as an Islamic national liberation movement, rather than a branch of the pan-Arab Muslim Brotherhood, which has been outlawed by Egypt. It also has dropped explicit language calling for Israel's destruction, although it retains the goal of eventually "liberating" all of historic Palestine, which includes what is now Israel.Israel considers Hamas a terrorist group and says it has not changed its ways.