Abdirahim Mohammed Abdullahi, one of the Islamic extremists who attacked Garissa University College, was the son of a government chief in Mandera County, Interior Ministry spokesman Mwenda Njoka told The Associated Press.

The chief had reported his son missing last year and said he feared that he had gone to Somalia, said Njoka.

All four attackers were killed by Kenyan security forces on Thursday, said police.

Abdullahi graduated from the University of Nairobi with a law degree in 2013 and was viewed as a “brilliant upcoming lawyer,” according to someone who knew him. It is not clear where he worked before he disappeared last year, Njoka said.

To prevent an escalation of Islamic radicalization in Kenya, it is important that parents inform authorities if their children go missing or show tendencies of following violent extremism, said Njoka. Somalia's Shebab Islamic militants claimed responsibility for the attack on Garissa University College Thursday saying it is retribution for Kenya deploying troops to Somalia to fight the extremist rebels.

The news that one of the attackers was Kenyan came as grieving Christians prayed, sang and clapped hands at an Easter Sunday service at a Catholic church in Garissa.

Security forces patrolled the perimeter of Our Lady of Consolation Church, which was attacked by militants almost three years ago. Grenades lobbed at the building sprayed shrapnel into the interior, injuring some worshippers. Another Garissa church was also attacked that day and 17 people were killed.

Sunday's ceremony was laden with emotion for the several hundred members of Garissa's Christian minority, which is fearful following the attack by Shebab, a Somalia-based Islamic extremist group.

The gunmen who attacked Garissa University College on Thursday singled out Christians for killing, though Shebab has a long record of killing Muslims over the years.

“Thank you for coming, so many of you,” Bishop Joseph Alessandro said to the congregation. He said some of those who died in Thursday's attack would have been at the service, and he read condolence messages from around the world.

The Islamic militants said the attack on Garissa college was in retaliation for killings carried out by Kenyan troops fighting the rebels in Somalia. “No amount of precaution or safety measures will be able to guarantee your safety, thwart another attack or prevent another bloodbath,” said Shebab.

Following the extremists' threats, Kenyan President Uhuru Kenyatta vowed to take harsh measures against the Islamic militants. In a nationally televised address, Kenyatta said his administration “shall respond in the severest ways possible” to the Garissa attack, in which four gunmen entered the campus and slaughtered students.

The military moved in hours later and the gunmen were killed. “We will fight terrorism to the end,” said Kenyatta. “I guarantee that my administration shall respond in the fiercest way possible". Kenyatta said the country's “security forces are pursuing the remaining accomplices."

"We will bring all of them to justice ... We are also in active pursuit of the mastermind (of the Garissa attack) and have placed a reward for his capture,” said Kenyatta, who declared 3 days of national mourning.

Five people have been arrested on suspicion of involvement in the Garissa attack, a Kenyan official said.