Story highlights It remains unclear who is behind the Sunday morning explosion

The blast comes two days after deadly bombings in Giza

(CNN) Security fears swept across Cairo on Monday as police probed the deadly bomb blast that ripped through a Coptic church the day before.

The explosion Sunday morning -- in the city's Abbassiya district -- hit the small church of St. Peter and St. Paul attached to the St. Mark's Coptic Orthodox Cathedral, said the Coptic Church's official spokesperson, Rev. Paul Halim, according to the government-sponsored Al-Ahram news outlet.

At least 25 people are dead and dozens injured, Al-Ahram said, citing the health ministry.

Egyptian state news agency MENA reported, citing security sources, said a 12-kilogram TNT bomb caused the blast..

In response to questions from CNN about the size of the bomb, a police official who asked not to be named told CNN: "It is too early to determine the type and amount of the explosives used," and authorities are unable to confirm reports about a 12-kilogram bomb.

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