The New Year's Day bombing outside a church in Egypt was officially blamed on "foreign hands" in the immediate aftermath of the attack.
Police now say they are looking at the possibility that home-grown extremists were involved - but many Coptic Christians already suspected it.
"I think those responsible are Egyptians but they are extremists, they are brain-washed," says Nadia Takla, a Christian visiting the injured in Alexandria.
Sectarian relations were frayed long before the blast that killed more than 20 people in Alexandria.
"All Christians expected something here in Egypt," says Ekramy Edward, a surgeon who operated on victims. "Every day we have troubles."
The Egyptian Initiative for Personal Rights (EIPR) has reported a recent rise in violent incidents involving Copts and Muslims.
It recorded 52 cases from 2008 to 2010. None of the perpetrators were brought to justice, it added.
"This [bombing] is different from other attacks we've seen, but I can't say we were entirely surprised," says Soha Abdulatty, a spokeswoman for EIPR.
"Tensions have been boiling and were due to spill over at any point."
Until Saturday, the worst attack in a decade had been on Coptic Christmas Eve in 2009. Six Christians and a Muslim security guard were killed in a drive-by shooting after mass.
Then, in November 2010, two protesters were killed in clashes with security forces near Cairo over plans for the construction of a church.
Other recent sectarian clashes have been sparked by disputes between neighbours that have escalated to draw in entire communities divided along religious lines.
The publication of controversial books and mere rumours of interfaith relationships or sexual assaults have also led to violent attacks.
Conversions - which the government only recognises when the person converts from Christianity to Islam, and not vice versa - are a particularly sensitive issue.
Reports that two Egyptian women, both priests' wives, had been allegedly hidden by the Coptic Church to stop them converting to Islam, were picked up by Islamist groups.
They staged demonstrations demanding their release, and the cause was then used by al-Qaeda in Iraq to justify assaults on Christians there.
Extremist websites urged Muslims to hit back at churches in Egypt around Christmas, which falls on 7 January in the Coptic Orthodox calendar.
"The operations are to be continued, with God's will, unless you release our chaste and pure sisters," one stated after Saturday's explosion.
The church denies holding the women, but Copts have been known to convert to Islam or another Christian denomination because of the Coptic church's tough policy on divorce.
Since the 1970s, Egypt has seen a rise in Islamism. Egyptian Christians - who make up 10% of the population - often complain of being second-class citizens.
"The Copts suffer from unequal legislation which makes it harder for them to build places of worship, they cannot occupy high-ranked posts in all state bodies, and were marginalised in candidate lists for the ruling party in the last election," says Yousef Sidhoum, editor of the Coptic newspaper, al-Watani.
"The Copts are growing more and more angry and that is why they are more vocal," he adds.
Divisive decisions by public servants and courts add to the Copts' list of grievances.
In 2009, the authorities ordered the destruction of 250,000 pigs as a questionable measure against swine flu.
This damaged the livelihoods of Cairo's large community of zabaleen - Christian rubbish collectors who use the animals to dispose of organic waste.
Many saw the cull as a veiled Muslim expression of disgust at pigs.
In Egypt, family law is based on Islamic Sharia law principles. As a result, Copts who have tried to adopt children in a way not recognised by Islam, have ended up in jail.
In a highly publicised custody battle two years ago, a Coptic woman also lost custody of her 14-year-old twins to their estranged father who had converted to Islam.
The dramatic scale of the weekend's attack has increased pressure on the authorities to confront a problem they have often ignored.
The Egyptian government and religious leaders have responded to the Alexandria explosion by calling for national unity, but campaigners say it will take more work to mend divisions.
"There needs to be an understanding of the sentiments that have led to this situation in society," says Ms Abdulatty.
"A much more comprehensive approach needs to adopted. It's not just a clash between a few individuals for the security forces to handle. It's a much bigger problem."