Interior ministry spokesman Mwenda Njoka said high-flying Abdirahim Abdullahi 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 an A-grade student, Njoka said it was "critical that parents whose children go missing or show tendencies of having been exposed to violent extremism report to authorities".
Hundreds had packed Nairobi's Anglican cathedral on Sunday, where Archbishop Eliud Wabukala said Easter services were overshadowed by "great and terrible evil" as police patrolled outside.
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"These terrorists want to cause divisions in our society, but we shall tell them, 'You will never prevail'," the archbishop said.
Somalia's Shebab militants attacked the university in the northeastern town of Garissa at dawn on Thursday, lining up non-Muslim students for execution in what President Uhuru Kenyatta described as a "barbaric medieval slaughter".
He said people's "justified anger" should not lead to "the victimisation of anyone" -- a clear reference to Kenya's large Muslim and Somali minorities in a country where 80 percent of the population is Christian.
The massacre, Kenya's deadliest attack since the 1998 bombing of the US embassy in Nairobi, claimed the lives of 142 students, three police officers and three soldiers.