Cairo - The massacre of more than 300 worshippers at a mosque in Egypt's Sinai crossed a new line - even by militants' brutal standards - and could be a sign the Islamic State group is trying to make up for the loss of its "caliphate" in Iraq and Syria or that an even more ultra-extremist faction is rising in power.
Either way, if the ISIS affiliate in Sinai does have a new readiness to slaughter Muslims, that threatens to put a further strain on Egypt's security forces and intimidate anyone cooperating with the government in the fight against militants.
But it also could raise a backlash against ISIS, prompting Sinai tribes to cooperate with the military and take greater action to stop any of their members from joining the group.
The ISIS-linked militants waging a campaign of violence in the Sinai and other parts of the country the past three years have traditionally targeted security forces, government officials, Christians and Muslim civilians suspected of collaborating with authorities. However, the November 24 attack - the bloodiest ever militant attack in Egypt - hit ordinary Muslims gathered for a Friday sermon, followers of the mystical movement in Islam known as Sufism that militants view as heretical.
"The ceiling of who is an infidel has risen to include worshippers and to view the slaying of Muslims inside mosques as permissible," said Ahmed Ban, an Egyptian expert on Islamic extremist groups.
Ban suspects that followers of an ultra-extremist ISIS faction known as "Hazimiyoun" played a role or were behind the mosque attack.
It may also be possible that other ISIS militants carried it out, feeling pressure from the Hazimiyoun faction's rising power to show they are not "soft."
There has been no claim of responsibility for the Sinai attack, and it is impossible to confirm that the faction did have a role.
Some experts believe that ISIS may have felt it needed a "showcase" attack to show it remains deadly even after losing almost all its territory in Syria and Iraq.
Tore Hamming, a researcher at the European University Institute focusing on jihadis and ideological differences within ISIS, said the mosque butchery was not necessarily connected to the Hazimiyoun faction.
No ISIS fighters "would consider Sufis true Muslims." He believed the attack came from a "great need (among ISIS) for large symbolic attacks."
Even before the attack, Egyptian newspapers reported the emergence of the Hazimiyoun faction in Egypt. One quoted a prosecution official saying detained Egyptian ISIS suspects told their interrogators that they are followers of the Hazimiyoun and consider some ISIS leaders as infidels.
The faction is named after a radical cleric, Ahmad bin Omar al-Hazimi, who has been imprisoned in his home country of Saudi Arabia since 2015.
It considers as infidels - and therefore as legitimate to kill - all Muslims who do not accept the Islamic State group's interpretation of Islam.
Even further, it says those who don't consider such people as infidels are also infidels deserving of death. Al-Hazimi himself is not known as an ISIS member but his ideology has gained support within ISIS ranks.
The Islamic State group is notorious for atrocities in Iraq, Syria and elsewhere, including against fellow Muslims; the group argues that killings of Muslims are justified when they were fighting ISIS or cooperating with its enemies or belong to branches of Islam it rejects, like Shiism.
In Yemen, four ISIS suicide bombers struck two mosques filled with worshippers, killing over 130 people in one day in March 2015.
But ISIS largely argued that Muslims in general, even if they haven't sworn allegiance to ISIS, are not necessarily legitimate targets, on grounds of "ignorance" - namely, that they may not have the religious knowledge to accept ISIS. The Hazimiyoun faction rejects the "ignorance" excuse.
The feud within ISIS has been stoked by the group's military defeats in Iraq and Syria. Hazimiyoun clerics have blamed ISIS leader Abu Bakr al-Baghdadi and his "lenient" ideology for the setbacks.
But the tensions date back to 2015, when ISIS in Syria executed a number of pro-Hazimiyoun clerics because of their excesses in declaring others infidels.
The more radical free hand has been criticized by al-Qaeda-linked groups and other extremists in Egypt.
Another militant group fighting the government in Sinai, called Jund al-Islam, said in a recent audio message that it had ambushed an ISIS patrol to avenge attacks on other Muslims.
Whether due to growing Hazimiyoun influence or not, the first major militant attack on a Muslim congregation in Egypt underscored that ISIS could dramatically expand its targets.
That could strain Egypt's security forces, already stretched thin across much of the country to protect government facilities, major thoroughfares and tourist hotels, as well as banks, churches and museums.
The militants have targeted individual Sufis in the past. Extremists consider some Sufi practices to be a form of idolatry and heresy, but millions of Egyptians follow Sufism, and it is considered part of mainstream Islam.
The targeted mosque was the headquarters of a local Sufi order, but non-Sufis were also among the 305 killed last week, including workers from a nearby salt factory.
"Any segment of society now is game for them," said Mohammed Gomaa, an expert on extremist groups from the Al-Ahram Center for Political and Strategic Studies.
He said the mosque attack was "certainly the work of one of ISIS factions. They are sending a message of how far they're willing to go, frighten tribes into submission and provide a response to a series of battlefield successes by security forces."
But the attack could instead anger the tribes. Most of the victims were from Sinai's powerful Sawarkah tribe, from which many of the ISIS affiliate's fighters are believed to hail. The government hopes that will prompt the tribes to isolate the militants. Some tribal leaders have called on the government to arm them to fight ISIS, but security agencies have been reluctant.
After the attack, President Abdel-Fattah al-Sisi ordered the military and security forces to crush the insurgency within three months, telling them to use "brute force."
Fawaz Gerges, an expert on extremist groups who lectures at the London School of Economics, believes the scale and brazenness of the mosque attack could portend more brutal and indiscriminate attacks by the militants - and prompt a similar escalation by security forces, which responded to past major militant attacks by, according to local rights activists, collective punishment or carpet bombing.
He said it appeared the ISIS affiliate or rogue elements within it have decided to "wage all-out war, not just against the Egyptian state, but the Egyptian people as well."
"It's a doomsday scenario, and I believe there is no turning back now," Gerges added. "The Egyptian state will win at the end, but the question is at what cost and how long will it be before it does."