Christians across Kenya sang the national anthem before Easter Sunday services in a message of defiance aimed at Islamist militants who killed almost 150 students last week, as the nation reacted with shock at the news that one of the gunmen had been a young Kenyan law graduate.

The interior ministry named the gunman as Abdirahim Abdullahi, a formerly straight-A student who received his degree from the University of Nairobi, Kenya’s most prestigious law school, before slipping into Somalia.

His alleged involvement in the Garissa attack is all the more striking because his father is a chief – a government official – in Mandera, a town in Kenya’s north-east near the border with Somalia. Chiefs are officials retained by the national authorities to solve disputes at local level and their remit includes identifying criminals.

“The father had reported to security agents that his son had disappeared from home and was helping the police to try to trace his son by the time the Garissa terror attack happened,” said Mwenda Njoka, a spokesman for the interior ministry.

Red Cross staff console a woman in Nairobi after she viewed the body of a relative killed in Thursday’s attack. Photograph: Khalil Senosi/AP

Abdullahi Mohammed, who presides over Bula Jamhuri village near Mandera, spoke to the Guardian briefly before he was rushed to hospital, apparently overwhelmed by the reports of his son’s involvement in the massacre.

“This is a young man who disappeared in 2013 and I have not heard from him since,” he said. “If it is true and it is confirmed that he was one of them, then he deserves the most painful death. This is so inhuman what they did to the innocent students. It is so wrong.”

All four al-Shabaab gunmen were killed after the attack, in which they stormed the Garissa university at dawn on Thursday before lining up Christian students in their hostels and spraying them with bullets. Twenty two students who were attending morning devotion were killed after several grenades were lobbed into their makeshift chapel.

The final death toll after the deadly rampage was given by authorities as 148, comprising mainly students between the ages of 19 and 23. Six security officers were killed trying to end the 12-hour siege.

In an address to the nation on Saturday evening, president Uhuru Kenyatta promised security forces would respond to the atrocity in the “severest way possible”, while seeming to reference the fact that Kenyans had been among the killers.

“Our task of countering terrorism has been made all the more difficult by the fact that the planners and financiers of this brutality are deeply embedded in our communities,” Kenyatta said.

“Radicalisation that breeds terrorism is not conducted in the bush at night. It occurs in the full glare of day, in madrasas, in homes and in mosques with rogue imams.”

Kenyan authorities came in for searing criticism after the Sunday Nation, the country’s biggest newspaper, reported that the commandos who ended the siege had been delayed for hours at their base in Nairobi because no plane could be found to ferry them to Garissa.

“It beggars belief that many of the failures that were witnessed during the [2013] Westgate [shopping mall] siege – including the late deployment of specialised police – were repeated in Garissa,” the paper said in a strongly worded editorial. It added: “Top security officials who failed in their duty owe Kenyans an apology.”

President Uhuru Kenyatta, flanked by deputy William Ruto, holds a news conference on Saturday after the massacre. Photograph: Thomas Mukoya/Reuters

As the recriminations flew, however, Kenyans put on a show of unity in many parts of the country. At a stadium in Nairobi where survivors of the attack waited for their parents to collect them, volunteers distributed cakes and poured out steaming milky cups of tea, a popular drink in Kenya.

Hundreds of Muslims staged anti-Shabaab demonstrations in Garissa and in Eastleigh, a busy trading centre in the capital known as Little Mogadishu due to its large population of ethnic Somalis.

The message from clerics during church services conducted under heavy guard was one of defiance and hope. “Even in a time of great distress, you should know God still loves Kenya,” said pastor Ambrose Nyangao of the Parklands Baptist Church in Nairobi.

There was praise, too, for members of the security forces, who risked their lives to save students.

“Courage is not a belief in invincibility, silly messianism, thoughtlessness or reckless risk-taking,” said Catholic bishop Anthony Muheria. “Rather, it is a deep awareness of one’s vulnerability and a willingness to still face the danger squarely, for a greater good.”

Others, though, demanded that Muslims confront the problem of radicalisation within their communities.

The prominent cleric David Oginde, writing in Kenya’s Sunday Standard said communal tensions would boil over if attacks continued.

“It is time Kenyan Muslims came out, not just in condemnation of attacks, but in a long-term [campaign of] education of children and youth, so that they do not fall easy prey to extremists … unless concerted efforts are taken, this nation is headed towards a religious war … a very frightening prospect indeed.”