Authorities in Kenya have said they have identified one of the four dead al-Shabaab gunmen who massacred nearly 150 people at Garissa University as an ethnic-Somali Kenyan national and law graduate.

Interior ministry spokesman Mwenda Njoka named one of the attackers as Abdirahim Abdullahi, saying he was "a university of Nairobi law graduate and described by a person who knows him well as a brilliant upcoming lawyer."

The spokesman said Abdullahi's father, a local official in the northeastern county of Mandera, had "reported to the authorities that his son had gone missing and suspected the boy had gone to Somalia".

Describing Abdullahi as a high-flying A-grade student, Mr Njoka said it was "critical that parents whose children go missing or show tendencies of having been exposed to violent extremism report to authorities."

This news comes as Kenya began three days of mourning for the victims of the attack.

President Uhuru Kenyatta has vowed to respond "in the severest ways possible" to the al-Shabaab militant attack.

Mr Kenyatta also said the government would take steps to crack down on those who planned and financed terrorist attacks.

Guarded Easter celebrations in Kenya

Following Thursday’s attack churches in Kenya used armed guards to protect their Easter Sunday congregations.

At one church in Mombasa, worshippers were evacuated and a bomb disposal unit deployed due to a suspicious vehicle parkeed outside. Police took the vehicle away for examination.

Kenya's Christians, who make up 83% of the population of 44 million, have been horrified by survivors' tales of how the militants sought out Christian students to kill while sparing some Muslims.

Yesterday Militants threatened to stage more attacks and turn Kenyan cities "red with blood". Police said they are providing extra security at shopping malls and public buildings in the capital Nairobi and in the eastern coastal region.

Kenyan priests, who have been targeted by Islamists in the past, say they fear Christian churches may bear the brunt of possible fresh attacks on Easter Sunday.

"We are very concerned about the security of our churches and worshippers, especially this Easter period, and also because it is clear that these attackers are targeting Christians, "Willybard Lagho, a Mombasa-based catholic priest and chairman of the Coast Interfaith Council of Clerics (CICC), told Reuters.        

He said Christian churches in the Indian Ocean port city of Mombasa will be hiring armed policemen and private security guards for masses today.