A horrific coordinated suicide bomb attack on two Shiite mosques in Yemen’s capital city of Sanaa has killed at least 100 people and injured hundreds more, according to Fox News. ISIS militants have claimed responsibility for the attack, which targeted members of the Iran-backed Shiite Houthi tribe which overthrew Yemen’s American-backed Sunni elected government in January.

The explosions were timed to coincide with midday prayers, guaranteeing maximum carnage at the crowded mosques. Multiple suicide bombers were used to carry out the attack, with a fiendish strategy of using a second bomb detonated outside the mosque to kill people fleeing from the initial explosion inside.

Surviving witnesses report seeing a total of four bombers, but the Islamic State said there were five in its online statement, which described the massacre as a “blessed operation” against the “dens of the Shiites.” The Associated Press notes this statement came from the same website that posted ISIS’ claim of ordering Wednesday’s shooting spree at a museum in Tunisia.

The New York Times reports that a third mosque was targeted, perhaps by the fifth bomber ISIS described, but he was detected and stopped before he could strike, blowing himself up without causing any other fatalities.

The Yemeni Interior Ministry said the death toll from the blasts would likely continue to rise; the NYT quotes an estimate of “at least 60 people killed at each mosque,” which would make it substantially higher than the 100 fatalities widely reported at the time of this writing. Yemen’s spokesman in Washington said that at least 119 people were killed and over 350 injured, according to NBC News.

The New York Times report also cites witnesses who said at least twelve members of a single large family were killed, and adds the detail that one of the bombers smuggled explosives in a fake cast on his leg. The ruse evidently failed to get him past a security checkpoint, so he detonated further away from the mosque entrance than he wanted, doubtless reducing the hideous death toll a bit.

Yemen has long been torn by an ugly power struggle between the elected government, Shiite rebel factions led by the Houthis, and al-Qaeda.  If ISIS is really behind the mosque massacres, it looks like they want a piece of the action in Yemen too, a disturbing sign of growing influence and territorial ambition from a terror state that President Obama vowed to “degrade and ultimately destroy.”

Obama also pointed to Yemen as an example of success in his foreign policy, just weeks before the Houthi rebellion kicked into high gear. Here’s how his big foreign policy success was looking, on the eve of the mosque massacre, per the Fox News report:

The bombings came a day after deadly clashes in the southern city of Aden, between forces loyal to President Abdrabbuh Mansour Hadi and the Houthi supporters of his predecessor, Ali Abdullah Saleh. Warplanes later targeted the presidential palace in Aden, where Hadi has been holed up since fleeing Sanaa last month. Hadi, who aides said was evacuated to a ‘safe place’ after the air raid, called the Aden attack part of a ‘failed military coup against constitutional legitimacy.’

As for former president Saleh, he’s still a player, and is reportedly aligned with the Houthis, despite having ordered military operations against them when he was in power.