The region has long been the scene of a simmering Islamist insurgency, and fighters loyal to Islamic State have mounted large-scale attacks there against the country’s security forces. But attacks against Muslim places of worship are rare in Egypt, and the scale of the bloodshed sent shock waves across the country.

The assault, which reportedly involved more than a dozen gunmen who arrived in off-road vehicles, targeted a mosque frequented by Sufi Muslims, followers of a mystical branch of Islam that Islamic State and other Sunni extremists deem to be heretical.

There were conflicting accounts of how the attack unfolded. Some witnesses said a bomb was detonated at the mosque’s main gate, prompting a panicked flight for the exits. Others did not report hearing an explosion but said at least three attackers broke into the mosque, locked the doors and started firing on those inside.

As worshipers tried to flee, more gunmen were lying in wait.

“The mosque has two exits, and both were covered by the terrorists, who were parked in cars and started firing at everyone who tried to escape,” said a local resident who wasn’t inside the mosque during the attack but rushed there to help. Like others interviewed, he asked to be identified by only his first name, Tariq, for fear of retribution.

“It was a horrible scene; there were bodies everywhere,” said another witness, Mahmoud, who also arrived soon after the attack and spoke with survivors. “They first shot at those praying inside, then onto the ablution area and killed those [in there] as well.”

The assailants blocked access to the area by blowing up cars in the road and also fired on ambulances as they rushed to the scene, witnesses and news reports said.

A video taken from inside the mosque showed the grim aftermath of the attack. Dozens of bodies were arrayed on the carpeted floor, their arms folded and their faces covered with sheets, many of them streaked with blood.

In addition to the 235 people killed, at least 109 were injured, according to state-run media. Egypt’s president, Abdel Fattah Sisi, declared three days of national mourning and convened a meeting of top security officials.

He later addressed the nation on television, vowing to respond with “extreme force” to this “painful, treacherous and cowardly” attack.

“This incident won’t do anything except increase our determination and strength to combat terrorism,” Sisi said.

Hussam Rifai, a member of parliament from Arish, told the local Masrawy news outlet that most of the casualties were from the Sawarka tribe, Sufis who are seen as broadly supportive of the government and who have refused to cooperate with Islamic State.

Residents said masked militants showed up at the mosque earlier this year and warned worshipers to stop helping the police.

Islamic State’s local affiliate has targeted Sufis and their shrines in the past. A year ago, the group published a video purportedly showing the beheading of Suleiman Abu Haraz, the 98-year-old spiritual leader of the Sufis in the Sinai who had been kidnapped from his Arish home.

Islamist militants have stepped up attacks in the region since Egypt’s military overthrew the country’s elected Islamist president, Mohamed Morsi, amid a wave of anti-government protests in 2013.

Hundreds of police, soldiers and civilians have been killed, most of them in attacks carried out by a group that pledged loyalty to Islamic State in 2014. The 47-mile stretch of road connecting Arish to Bir Abed has become a regular escape route for the militants, who consider the Sinai one of the provinces of their self-proclaimed caliphate.

“Historically the state has struggled to exercise full control over northern Sinai,” said Timothy Kaldas, a nonresident fellow at the Tahrir Institute for Middle East Policy in Washington. “The area is underdeveloped and is mostly mountainous desert terrain.”

Islamic State’s affiliate has carried out deadly bombings on churches in the capital, Cairo, and other cities. It also is believed to have been responsible for the downing of a Russian passenger jet that killed 226 people in 2015.

But the targeting of a mosque represents a shift for the militants, who typically aim their attacks at the security forces and people they view as spies or heretics.

Kaldas said the government would need to reach out to the local Bedouin tribes and enlist them in the fight against Islamic State.

“The government has started to do that, and we’ll see if this attack makes Bedouin in Sinai more interested in fighting ISIS given how this attack targeted them so brutally,” Kaldas said, using a common acronym for Islamic State.

Although Islamic State has lost control of most of the territory it once held in Syria and Iraq, security experts have warned that the group remains active in other parts of the world, and its followers probably will revert to guerrilla-style tactics.

The group’s Sinai affiliate is considered to be one of its most effective. Although it does not control territory, it has continued to inflict deadly attacks despite years of fighting with Egypt’s security forces.

Messages of condolence and support poured in from around the world Friday, including from Britain, France, Israel, Russia and the United Arab Emirates.

President Trump condemned the “horrible and cowardly terrorist attack on innocent and defenseless worshipers” in a tweet.

“The world cannot tolerate terrorism, we must defeat them militarily and discredit the extremist ideology that forms the basis of their existence!” he said.