European countries' reluctance to investigate may have something to do with the widely held belief that the killing of al-Mabhouh was carried out by a friendly country's intelligence agency - Israel's Mossad. The Jewish state has previously identified him as the point man for smuggling weapons to the Gaza Strip's Hamas rulers.

Experts say arresting Israeli agents - or even digging up further evidence that Israel was involved - could be politically costly. "I would guess that it's in the political interest of certain countries not to get proactive in this case," said Victor Mauer, deputy director of the Center for Security Studies at Zurich's Federal Institute of Technology.

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