The shooting death of Ali Habibi-Mousavi in Tehran has all the earmarks of a political assassination. The nephew of opposition leader and recent presidential candidate Mir-Hossein Mousavi reportedly had received death threats before he was shot in the heart by men believed to be security forces or pro-government militia. On Monday, Habibi-Mousavi’s family said his body was seized from the hospital, apparently to prevent them from holding a funeral that could ignite more protests -- a cycle that served Islamic revolutionaries when they toppled the shah 30 years ago.

The Iranian government denies killing demonstrators and claims that “foreign terrorists” murdered Habibi-Mousavi. Certainly the last thing it wanted was to create another opposition martyr alongside Neda Agha-Soltan, the student whose fatal shooting was captured on video last summer when the protest movement began as an outcry against election fraud. And yet that is quite possibly what happened on the holiest Shiite Muslim holiday, Ashura, honoring the martyrdom of Imam Hussein, a grandson of the prophet Muhammad who was killed fighting injustice.

Throughout the past six months, the Islamic government has used seemingly measured force and surgical arrests to try to put down the protests without fueling more opposition. But the protests are not dying out. On the contrary, photographs posted on websites and printed on front pages reveal emboldened demonstrators beating back government security forces, throwing rocks, attacking a police station and setting fires. The opposition appears to have grown into a politically and geographically diverse grass-roots uprising. Its challenge of the election results has expanded into a challenge to the very legitimacy of the Islamic government.

Against that backdrop, Habibi-Mousavi’s death, along with the killing of at least seven other protesters, seems a sign of increasing desperation on the part of government forces. Until now, the Ashura holiday was sacrosanct, a day for peace, not violence; as another former presidential candidate, Mehdi Karroubi, noted, even the deposed Shah Mohammed Reza Pahlavi had respected it. Hundreds of demonstrators were rounded up Sunday and 10 opposition leaders were arrested Monday, including former Foreign Minister Ebrahim Yazdi and three advisors to Mousavi. Those are the acts of a government tightening its grasp for fear of losing its grip altogether. President Obama condemned “the violent and unjust suppression” and called on Iran to abide by international human rights obligations. Indeed, by refusing to respect the rights of the opposition, the Iranian regime courts civil war.