Armed attackers killed at least 235 people in a mosque in Egypt's Sinai Peninsula, according to state media. The attack, which took place in the town of Bir al-Abed, about 40km west of the provincial capital of North Sinai, El Arish, happened shortly after Friday prayers.
Egyptian President Abdel Fattah el-Sisi called the attack "criminal" and "cowardly" in a televised statement on Friday.
Hours after the attack, Egyptian warplanes took to the skies, targeting the mountainous areas around Bir al-Abed.
Here is what we know so far.
During the Friday prayers at a Sufi mosque in Bir al-Abed, North Sinai, four 4WD vehicles drove up to the mosque. Reports said the attackers planted explosives and then opened fire on worshippers.
The attackers next targeted fleeing worshipers with gunfire.
At least 235 people were killed and 120 wounded, according to Egyptian state media. It was the deadliest attack of this kind in Egypt's history.
Images circulating on Twitter and shown on Egyptian TV showed dozens of bodies covered in blood, lying on the floor of the mosque.
Most of their faces were covered with white cloths, while other bodies were wrapped in prayer rugs. Some men and women could be seen next to the bodies.
The Bir al-Abed mosque is believed to have been an easy target because it was outside the province's main cities.
Another possible reason the mosque was targeted was because it followed a Sufi sect. Sufis are considered infidels by groups such as Islamic State of Iraq and the Levant (ISIL, also known as ISIS).
However, Friday's prayers were most probably attended by non-Sufis as well.
The incident came one day before the Egypt-Palestine border crossing at Rafah was supposed to open. The crossing had been due to open for three days beginning on Saturday, after being closed last August.
However, Egypt has said the border will now remain closed due to security concerns.
Who is behind the attack?
So far, no group has claimed responsibility but local affiliates of ISIL, also known as ISIS, have claimed previous attacks.
Timothy Kaldas, a professor at Nile University in Cairo, told Al Jazeera the attack "fits the pattern of ISIS attacks".
"Potentially, it's another attack against Sufis in northern Sinai. Potentially, it's retaliation for tribes co-operating with the state in the crackdown on ISIS," he said.
The Sinai Peninsula has been the scene of attacks for years, as Egypt has been battling an armed anti-government campaign in the rugged and thinly populated region.
The conflict ignited in 2013, after Egypt's military overthrew Mohamed Morsi, the first democratically elected president of Egypt after the Arab Spring. With the overthrow of Morsi, forces in the Sinai Peninsula rose up and started attacking Egyptian security forces.
Over the years, most attacks were targeted at soldiers and police, but civilians have been killed before as well. Hundreds of people have died in the conflict so far.
In 2014, President Sisi declared a state of emergency in the peninsula, after a suicide bomber killed 33 soldiers. He described the region as a "nesting ground for terrorism and terrorists".
During the last couple of months of 2017, a number of attacks have taken place, with six security forces killed in October and 18 in September.
How has the Egyptian government reacted to the attack?
Shortly after Friday's attack, President Sisi condemned the attack. He expressed condolences to the victims and their families and said the attack "will not go unpunished".
The government declared a three-day period of mourning for the victims.
"The armed forces and the police will avenge our martyrs and restore security and stability with the utmost force," Sisi said.
After Sisi's speech, Egyptian warplanes targeted several places in the area around Bir al-Abed, witnesses reported.
Kaldas said many people have criticised the Egyptian government's "scorched earth strategy" in Sinai, but conceded that the area is "very challenging terrain".
"It's a mountainous desert area that's not very developed. Even with the best strategy, it's a difficult place to control," he said.
Al Jazeera's Middle East analyst Yehia Ghanem said the intensified violence in Sinai over the last four years has been used by the Egyptian government to escalate repression throughout the country.
"The Egyptian regime has initiated violence in the Sinai - all over the country, but specifically in Sinai. It's a rule: violence begets violence," he said.
How has the world reacted to the Sinai attack?
Leaders and organisations around the world have spoken out en masse against Friday's attack.
The UN Security Council and Antonio Guterres, UN secretary-general, strongly condemned the attack. A statement released by the organisation called it a "heinous and cowardly terrorist attack" and reiterated that all acts of terrorism "are criminal and unjustifiable, regardless of their motivation."
US President Donald Trump condemned the massacre as "horrible and cowardly".
"The world cannot tolerate terrorism, we must defeat them militarily and discredit the extremist ideology that forms the basis of their existence!," he wrote on Twitter.
Later, he said he would call Sisi to discuss the attack and said "we have to be much smarter and tougher than ever", doubling down on his plan for a border wall and travel ban.
Pope Francis released a statement offering condolences to Egypt, saying "His Holiness joins all people of good will in imploring that hearts hardened by hatred will learn to renounce the way of violence that leads to such great suffering, and embrace the way of peace".
EU President Donald Tusk said on Twitter he was "appalled by the attack on a mosque in North Sinai. My thoughts are with Egypt and all those affected by this evil and cowardly act".
Jens Stoltenberg, NATO secretary-general, called the attack "barbaric". British Prime Minister Theresa May described it as an "evil and cowardly act".
In a speech in Istanbul, Binali Yildirim, Turkish prime minister, said: "235 innocent people were killed, there are hundreds of others injured. Is this Islam? Is this humanity?"