Islamist militants have been operating in northern Sinai for several years, mainly targeting security forces.
But this is the first time that worshippers inside a mosque have been targeted. The number of victims is unprecedented for an attack of this type.
Northern Sinai has been living under a media blackout over the past few years. No media organizations have been allowed to go there, including state-sponsored ones.
The frequency of attacks raises doubts about the effectiveness of military operations. While the army releases statements every now and then, claiming victory in some parts of Sinai, there seems to be no end in sight to the ongoing battle between the army and militants.
Arab League chief Ahmed Aboul Gheit condemned the attack as a "terrifying crime which again shows that Islam is innocent of those who follow extremist terrorist ideology".
UK Prime Minister Theresa May tweeted: "Appalled by the sickening attack on a mosque in North Sinai. Condolences with all those in #Egypt affected by this evil and cowardly act."
US President Donald Trump called the attack "horrible and cowardly". French Foreign Minister Jean-Yves Le Drian expressed condolences to these families of those killed in the "despicable attack".
Register for free to join our community of investors and share your ideas. You will also get access to streaming quotes, interactive charts, trades, portfolio, live options flow and more tools.