The investigation into a church bombing in Alexandria, Egypt, that killed 21 people is focusing on a local group of Islamic hard-liners who are thought to be inspired by al Qaeda.
Security officials say 25 people have been detained for questioning, but none is thought to be linked to the attack.
The officials told the Associated Press that the 25 were mostly owners of cars parked outside the church at the time, storekeepers and Muslim neighbors known to be Islamic fundamentalists.
Investigators were examining two heads found at the site on suspicion that at least one was the bomber's, Egypt's state news agency MENA reported.
The explosion outside a Coptic Christian church in Alexandria on Saturday is considered the worst violence against the country's Christian minority since the late 1990s.
Along with the 21 people killed, close to 100 others were injured. It's not known if all the victims were Christians.
The bomb exploded in front of the Saints Church about a half hour after midnight early Saturday, as nearly 1,000 worshippers were leaving a New Year's Mass.
The attack prompted Christian youths to protest on the streets, with some throwing stones at police who returned fire with rubber bullets.
No group has claimed responsibility for the bombing. President Hosni Mubarak has blamed "foreign hands" and Egypt's Interior Ministry said a foreign-backed suicide bomber might have been responsible.
The Alexandria governor accused al Qaeda, pointing to threats against Christians in Iraq by the terror network's branch in that country.
Egypt's government has long insisted that al Qaeda does not have a significant presence in the country. But Egypt has also seen a rising movement of Islamic hard-liners who adhere to an ideology similar in many ways to al Qaeda.
Alexandria, which a century ago was home to a mix of Muslims, Christians, Jews and foreigners, has become in the last decade a stronghold for Islamic hard-liners, known as Salafis.
However, officials believe that the attackers most likely came from a small fringe group even more radical than the Salafis.
On Sunday, dozens of grieving Christians returned to pray at the blood-spattered church, many of them sobbing, screaming in anger and slapping themselves in grief.
Inside the church, the floor was still stained with blood, statues of Jesus and the Virgin Mary were toppled and benches were scattered by the impact of the blast.
A wooden cross hanging on the church gate was covered with a white sheet stained with victims' blood and bits of human flesh remained stuck on the gate. Young Christian men prevented cleaners from removing the flesh.
The country's chief Muslim cleric, Grand Sheik of al-Azhar Ahmed el-Tayeb, met with the leader of the Coptic church, Pope Shenouda III, and offered condolences.
Shenouda asked for justice, and said that failing to do so would lead to "frustration."
Meanwhile, Pope Benedict condemned the bomb blast Sunday.
"This vile gesture of death, like that of putting bombs near to the houses of Christians in Iraq to force them to leave, offends God and all of humanity," the Pope said after his weekly angelus blessing.
The pope urged Christian communities to renounce violence, even in the face of what he described as "a strategy of violence that has Christians as a target."
With reports from the Associated Press
