Somali authorities have executed a 23-year-old man in Mogadishu for masterminding a bombing that killed about 600 people and injured hundreds more in the city exactly a year ago.

The explosion was one of the most devastating single strikes by Islamic militants in recent decades. Hassan Adan Isaq was shot by a firing squad early on Sunday, a Somali military judge said.

The convicted man was a veteran member of the al-Shabaab extremist group. He was arrested within days of the bombing and sentenced to death in February.

“He has killed hundreds as an al-Shabaab member and today he was rewarded with a painful death,” Hassan Ali Nur, the judge, said.

The execution will concern human rights campaigners. Somalia receives significant military assistance from the US and the UK. Death sentences have been carried out increasingly often in the unstable eastern African state in recent years, with 24 executions in 2017.

Investigators believe the target for the huge bomb, which was mounted on a truck and driven into Mogadishu before being stopped by police at a busy crossroads in the city centre, was a newly opened Turkish military training facility in the heavily guarded airport zone, where most foreign militaries and embassies are based.

The crossroads was very busy with students, workers and shoppers. The bomb detonated next to a fuel tanker and a minibus full of schoolchildren.

In Mogadishu, reaction to the execution was mixed. “This is not enough. Only one person was executed,” said Hasan Mohamud Ali, 21, a Mogadishu resident whose grandfather was killed in the attack. “Where are the others who were arrested for the carnage of 14 October?”

Anisa Mohamed Mohamud, 19, a university student who was injured in the bombing, said government’s investigation was inadequate. “We wanted more answers … where are the other accomplices who the security and police said they have apprehended? Why has the government not arrested the senior al-Shabaab guys for their crimes? A year has gone by and Mogadishu is still not safe,” she said.

Abdulahi Dahir Guled, 34, a bus driver whose wife died in the attack, said he had not received any government assistance. “As the families of the victims, we were expecting to get some support from the government. My children do not go to school. We are suffering because of that attack and nobody is helping us. The government did not give us even a single dollar,” Guled said.

Analysts say al-Shabaab, one of the world’s deadliest terrorist organisations, were concerned about public anger and did not make any public claim of responsibility for the attack. High-level defections from the organisation briefly increased following the bombing.

However, al-Shabaab has rarely worried about civilian casualties, inflicting thousands over its decade-long campaign to impose a harsh version of Islamic law on Somalia. A double bombing of a restaurant and a hotel in the town of Baidoa killed at least 16 people on Friday.

There has been a surge of US air strikes in support of Somali troops fighting al-Shabaab, with at least six such attacks in the last month.

The special representative of the UN secretary general for Somalia, Nicholas Haysom, called on al-Shabaab to refrain from using improvised explosive devices as a weapon.

“Owing to the indiscriminate nature of IED attacks, most of the victims are usually innocent civilians – men, women and children. No political purpose can ever be served by the random killing of civilians,” he said.

The bombing was described as “the Somali 9/11”. Witnesses described an area the size of two or three football fields where buildings had been razed.

Additional reporting by Abdalle Ahmed Mumin, Mogadishu