Civilians evacuate from the scene of an explosion in Mogadishu, Somalia. Feisal Omar/Reuters Officials in Somalia are still trying to determine the source of the single deadliest terrorist attack in the country's history after a truck bombing killed at least 300 people on a bustling road filled with traffic Saturday in the capital city of Mogadishu.

On Tuesday, hundreds more were recovering from injuries and many remained missing, hidden under the rubble of burned out buildings.

Somalia's government blames the Al-Shabab extremist group for the attack.

Al-Shabab, an Al-Qaeda affiliate, is Africa's deadliest Islamic extremist group and often targets high-profile areas of the capital. It has not yet claimed responsibility.

Such attacks are the norm these days in Mogadishu, which suffers from an average of two large explosions every month. More than 31 people were killed in an attack outside a popular hotel and restaurant in the capital in June. But this was the deadliest attack in years.

The massacre exposes Somalia's security vulnerabilities. Officials say attackers driving two vehicles — a Toyota minivan and a large truck carrying military-style and homemade explosives — were supposed to bomb a heavily guarded airport compound in Mogadishu, where international peacekeeping organizations, including the United Nations and the African Union, are based.

The driver of the minivan was stopped at a checkpoint and detained. The truck, meanwhile, detonated its bomb early after security guards at the checkpoint became suspicious. The Guardian reported that officials have been questioning the minivan driver to learn more about the motive behind the attack.

Families have already started to bury their dead. Farah Abdi Warsameh/AP

Through questioning, officials learned the man who set off the main truck blast was a former soldier in Somalia's army whose home town was raided by local troops and US special forces in August. Ten civilians, including three children, were killed in that operation. Officials say Saturday's bombing might have been revenge.

Officials also said the driver joined the army in 2010, but later defected to join the ranks of Al-Shabab.

Earlier this year, Al-Shabab said it would ramp up attacks after both the Trump administration and Somali President Mohamed Abdullahi Mohamed announced renewed military efforts against the group.

Al-Shabab and other extremist groups have been battling government and African Union forces in a bloody civil war for years, and security experts also point to them as the likely group behind the attack.

"No other group in Somalia has the capacity to put together a bomb of this size, in this nature," Matt Bryden, a security consultant on the Horn of Africa, told the Associated Press.

But the families of the 302 dead and 70 missing still don't have answers.