Updated on November 25 at 9:39 a.m. ET

Once death tolls get above 50 or so, they become impossible to process. The attack Friday on the Rawdah mosque in Bir al-Abed, north Sinai, killed more than 300 people, with a combination of bombs and light weaponry. The attackers executed survivors and gunned down first responders. Imagine five separate Las Vegas attacks happening all at once from different angles, starting while the victims are at prayer.

No one has claimed responsibility. But no points should be awarded for guessing that the Islamic State is at fault. It has motive and opportunity. Its Sinai province was the site in October 2015 of the downing of Metrojet 9268 out of Sharm al-Sheikh, which killed 224 people, mostly Russian tourists. The local IS affiliate, Ansar Beit al-Maqdis, has killed Egyptian police and soldiers in north Sinai, mostly around Al-Arish town, at a steady rate for the last three years. Most of these assassinations get no coverage in the western press. But IS propaganda has featured tidy little photo montages of men on motorbikes accosting plainclothes officers, shooting them in the head, and harvesting their ID cards (sometimes blood-smeared) for proof of death.

As for motive—the Islamic State loathes Muslims it considers to have left the religion. Apostasy, or leaving Islam, is its favorite crime: The most horrific propaganda involves killing of those the Islamic State considers ex-Muslims, such as Mu’adh al-Kasasbeh (the Jordanian pilot it burned alive), or the thousand or so Shia it killed in a single go at Iraq’s Camp Speicher in June 2014. Suspected adulterers merely get their heads bashed in. When the group murders apostates, the videos linger sadistically on the victim as he dances in a lake of fire, or the blood pumps out of a wedge cut from his throat.

The mosque attacked today in Sinai is associated with Eid al-Jarir, a Sufi saint and founder of an eponymous mystical order. For the Islamic State, mysticism—the belief in a direct relationship to the divine—is heretical, and veneration of saints in any form elevates humans to a status that can claim a piece of the total and indivisible lordship of God himself. These are both killing offenses. About a year ago in Sinai, Islamic State fighters beheaded a hundred-year-old cleric, Suleiman Abu Haraz, for propagating these heresies. It’s not clear whether the attendees of Friday prayers at this mosque were mostly Sufis. But the Islamic State has killed for milder theological offenses than praying at a Sufi mosque. (Just three days ago, it killed about 50 people at a mosque in Nigeria, with similar justification.)

Terrorist groups never give up easily. They fight for survival and relevance, and if they suffer losses—as the Islamic State has in Syria and Iraq—they feel the need to offset them with quick and spectacular massacres like this one. Absent a claim of responsibility from the Islamic State, of course, it’s possible that others have committed this slaughter and are seeking to make a name for themselves as the worst people on earth. But for the Islamic State, the motive and opportunity are accompanied by a special urgency. These 300 won’t be the last.