Gunmen from Somalia's al Shabaab terrorist group stormed the building on Thursday letting the Muslims go free and killing Christians.
Reports say the militants opened fire as unsuspecting Christians were praying with their backs turned away from the attackers.
Survivor Helen Titus told the Associated Press: "They investigated our area. They knew everything."
Authorities say 79 other people were hurt.
Kenyan Interior Minister Joseph Nkaissery has told CNN that five have been arrested after a reward of £148,370 was offered.
But it's not thought the alleged mastermind, former teacher Mohammed Mohamud, has been captured.
What is Al Shabab?
- Formed in 2006 in Somalia
- Means "The Youth" in Arabic
- Between 7,000 and 9,000 fighters
- In 2012 it pledged allegiance to Al-Qaeda
- Behind Nairobi's Westgate shopping centre attack
Meanwhile the last text message sent by a Christian student to her boyfriend has been revealed.
A screen shot of Jane Akinyi's text says: "In case we don't see each other again, just know I love you.
"Where are the KDF [Army]? If they are there, please tell them to come, they are killing us, please tell them to come help us.
"Bye babe, pray for us. May God help us."
Footage of the attack has since emerged appearing to show students running for their lives and hiding behind bushes.
Four gunmen were killed by the army and one was arrested as the 12 hour siege came to an end.
Pope Francis said the attack was an act of "senseless brutality".
In a telegram to the Church in the country he urged those responsible to change their ways and the Kenya authorities to work to "hasten the dawn of a new era of brotherhood, justice and peace".
At the Good Friday service Rt Revd John Njue, the Roman Catholic archbishop of Nairobi, said: "This is a tremendous challenge in our country."