Egypt's occasionally fragile sectarian ties have been shaken by the murder of a Muslim whose wife converted to Islam against her Christian family's wishes.
Police said Mariam Khilla's brother, Rami, broke into their Cairo home and opened fire, killing her husband and injuring Mrs Khilla and their daughter.
Mr Khilla had been searching for Mariam ever since she left home to marry Ahmed Saleh about two years ago, police said.
Muslim-Christian friction is unusual in Egypt but sometimes leads to violence.
Clashes are usually triggered by disputes over land, religious buildings or inter-marriage, correspondents say.
Police said Mariam Khilla and baby daughter Nora were critically wounded in the attack which took place overnight on Monday.
They are searching for Mr Khilla and his uncle who is alleged to have taken part in the attack.
Police sources said he had urged the couple to divorce and for Mrs Khilla to return to Christianity, but they had refused.
The killing came less than a week after a Christian man was killed in sectarian clashes in southern Egypt after a Muslim man was accused of flirting with a Christian girl.
Egyptian Christians - known as Copts - are the largest Christian community in the Middle East, making up an estimated 10% percent of Egypt's population of 80 million.