Gunman killed at least 26 people and wounded 25 other people Friday in Egypt.
“I call on Egyptians to unite in the face of this brutal terrorism,” Ahmed al-Tayeb, the grand imam of al-Azhar, said from Germany.
“The embassy is aware of a potential threat posted on a website by the Hassm Group.”
A group of Coptic Christians, an Orthodox Christian denomination that originated in Egypt, were traveling in two buses and a truck in Minya province.
“The bus was heading towards the Monastery of St. Samuel the Confessor in Minya province, about 135 km (85 miles) south of Cairo, when it came under fire,” the British Broadcasting Corporation reported.
The Christians, including children, were reportedly headed to pray.
“Only three of the children traveling with the worshippers survived, local media claimed,” The Daily Mail noted on Friday morning.
No group has yet taken responsibility for the attack.
“As many as 10 attackers stormed the bus dressed in military uniforms and wearing masks, according to witnesses,” Fox News reported. The gunmen stopped the vehicles and opened fire on the road.
The gunmen used automatic weapons to plaster the bus with bullets, according to reports.
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“Unidentified gunmen had arrived in three four-wheel-drive vehicles,” an interior ministry spokesman said, according to Reuters.
The U.S. Embassy and Consulate in Egypt issued a security warning to U.S. citizens earlier this week.
“The Embassy is aware of a potential threat posted on a website by the Hassm Group, a known terrorist organization, suggesting some kind of unspecified action this evening,” the U.S. Embassy in Cairo said in a message issued on May 24.
Egyptian Christians have been the targets of terrorism. Last month, radical Islamic extremists bombed two churches in Egypt on Palm Sunday.
“Among the wave of recent attacks on Egypt’s Christians: twin suicide bombings in April and another attack in December on a Cairo church that left over 75 people dead and dozens more wounded,” Fox News noted. “ISIS in Egypt claimed responsibility for them and vowed more attacks.”