Kenya claims to have identified one of the al-Shabaab gunmen who massacred university students in north-east of the country as the son of a government official, the interior ministry has said.

Spokesman Mwenda Njoka said Abdirahim Abdullahi was one of the four men who attacked the Garissa University college campus on Thursday, killing almost 150 people.

“The father had reported to security agents that his son had disappeared from home … and was helping the police try to trace his son by the time the Garissa terror attack happened,” Njoka told Reuters.

The claim came as Kenyan churches used armed guards to protect their Easter congregations. Priests, who have been frequently targeted by Islamists, said they feared churches could be targeted 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 added that churches in the port city were hiring armed police and private security guards for mass on Sunday. Christians account for about 83% of Kenya’s population of 44 million.

In Garissa, where four masked al-Shabaab militants stormed a university campus on Thursday, seeking out Christian students to kill while sparing some Muslims, six soldiers guarded the town’s main Christian church and about 100 worshippers before of Sunday mass.

“Nowhere is safe, but here at church you can be with God and console yourself,” said Meli Muasya at the town’s Catholic church. In 2012 masked gunmen killed more than a dozen people in simultaneous gun and grenade raids on two churches in the town.

“We just keep on praying that God can help us, to comfort us in this difficult time,” Dominick Odhiambo, a worshipper told the Associated Press, adding that he planned to abandon his job as a plumber and leave for his hometown because he was afraid.

“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.

Al-Shabaab said the assault in Garissa, which is 120 miles (200km) from the Somali border, was revenge for Kenya sending troops into Somalia to fight alongside African Union peacekeepers against the group.

The group is aligned to al-Qaida and has threatened to turn Kenyan cities “red with blood” with more attacks. Police have stepped up security at shopping malls and public buildings in the capital Nairobi, and in the eastern coastal region which has been prone to al-Shabaab attacks.

On Sunday, two uniformed police officers armed with AK-47 rifles manned the entrance gate to Nairobi’s Holy Family Basilica Cathedral. More plainclothes officers were inside, said one.

Three private security guards frisked churchgoers with handheld metal detectors, while a fourth guard used a mirror to check for explosives underneath cars.

“Everyone is anxious and you never know what will happen next, but we believe the biggest protector is God and we are praying,” said Samuel Wanje, 27, a youth member at the church.

Kenya has imposed a dusk-to-dawn curfew in four counties along its 435-mile (700km) border with Somalia and deployed helicopters along its coast, which is popular with western tourists and the target of Islamist attacks in the past.

The coastal region’s police chief, Robert Kitur, told Reuters extra uniformed and plainclothes officers had been deployed. “What happened in Garissa must never be seen in Mombasa or anywhere else in the region and country,” he said.

On Saturday, the Kenyan president, Uhuru Kenyatta, vowed harsh measures against the Islamic militants. “We will fight terrorism to the end,” he said in a televised address. “I guarantee that my administration shall respond in the fiercest way possible.”

But criticism is mounting over the response of the Kenyan special forces on Thursday, who, according to scathing reports on Sunday, took at least seven hours to deploy.

Alarm bells rang in Kenya’s elite Recce Company in Nairobi as soon as the first reports of the predawn attack emerged in Garissa. But it would take until just before 2:00 pm until the main team reached the attack site, Kenya’s Nation newspaper said, noting that the first aeroplane carried the interior minister and police chief.

“This is negligence on a scale that borders on the criminal,” the paper said in its editorial, recalling how survivors said “the gunmen, who killed scores of students with obvious relish, took their time”.

The Standard newspaper’s editorial cartoon accused security forces of sleeping on the job, depicting a snake labelled “terror threat” waking a snoring security officer with a bite, as a dog barks, “too little, too late”.

But the foreign minister, Amina Mohamed, defended the security forces’ response, telling AFP on Saturday: “Fighting terrorism … is like being a goalkeeper. You have 100 saves, and nobody remembers them. They remember that one that went past you.”