According to Hujjatollah Mojtaba Zolnour, Ayatollah Khameinei's deputy representative to the IRGC (Iranian Revolutionary Guards Corps), "Zionists are waiting for the Hidden Imam to appear to kill him" (Radio Free Europe's "Persian Letters," September 1, 2010: http://www.rferl.org/content/Zionists_Waiting_To_Kill_Hidden_Imam_Says_Iranian_Cleric/2145806.html?page=1&x=1#relatedInfoContainer).   Zolnour alleges that those wily, perfidioius Zionists have assassins staking out  the likes of Mecca and Karbala, ready to send the 12th Imam (back?) to the domain of the 72 houris.  Of course, according to the fevered Mahdist imaginations of the Zolnours in Tehran and Qom, Israelis are part of the occupying forces in Iraq--hence their ability to case Karbala.  
This paranoid fear for the life of Islam's major messianic figure is strange indeed.  First, we have heard this before: earlier this year Presdient Ahmadinejad claimed that the U.S. was "hindering the Mahdi's return"(http://www.alarabiya.net/articles/2010/02/24/101367.html); and Moqtada al-Sadr (he of southern Iraq's Shi`i Jaysh al-Mahdi, "Army of the Mahdi"), said the same thing in 2006 (http://www.timesonline.co.uk/tol/news/world/iraq/article658083.ece).  Second, who is the intended audience for this claptrap--the Basij (the IRGC's domestic, paramilitary auxiliary)? the rural hoi polloi? Iraqi Shi`is? the non-existent Palestinian Shi`i demographic? And third, do not clerics like Zolnour who utter such unadulerated silliness realize just how impotent and vulnerable it makes their expected deliverer appear?  Imagine Christian televangelist John Hagee or, much more seriously, Pope Benedict XVI claiming that the returned Jesus Christ could be hindered, much less killed, by, say, Marxist or "Zionist" hit teams. 
Yet for some reason influential Twelver Shi`i spokesmen (Zolnour, Ahmadinejad and al-Sadr) continue this refrain of the coming Mahdi's vulnerability to profane powers and their weaponry. 
It's almost enough to make one wonder whether these men might be laying the propaganda ground for the attempted assumption of the Mahdi's title by someone, and his inevitable subsequent demise at the hands of Americans, Sunnis or "unbelieving" Shi`is--er, I mean, "Zionists!"  Which would, of course, give the Islamic Republic of Iran an excuse to....well, fill in the blank.  But a live-fire test of a nuclear weapon certainly comes to mind.