The boys, who were playing in a lane near the mosque, were buried in the debris when the dome collapsed around 4 pm yesterday.
A huge crowd gathered at the spot, delaying rescue operations, said head priest Zainul Abideen.
While Saad was killed on the way to Delhi for treatment, the other two were seriously injured and were being treated at the District Hospital here.
According to Abdul Nafe Ansari, municipal corporator of the area, the mosque was about 800 years old.
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