MOGADISHU, Somalia (AP) — Somali intelligence officials shared a detailed account of the country's deadliest attack, while thousands marched Wednesday in Mogadishu in a show of defiance against the extremist group blamed for Saturday's truck bombing that left more than 300 dead.

Two people have been arrested in the attack that was meant to target Mogadishu's heavily fortified international airport, where several countries have their embassies, the officials said.

Somalia's president urged the long-fractured Horn of Africa nation to unite, and Mayor Thabit Abdi said the city was "awash in graves." Some desperate relatives still dug through the rubble with their bare hands in search of scores said to be missing.

Wearing red headbands, a crowd of mostly young men and women gathered at a Mogadishu stadium and shouted slogans against al-Shabab, which has long targeted the seaside city but has not commented on the attack.

Some in Somalia have called the bombing their "9/11," asking why one of the world's deadliest attacks in years hasn't drawn more global attention. Nearly 400 others were wounded.

"You can kill us, but not our spirit and desire for peace," said high school teacher Zainab Muse. "May Allah punish those who massacred our people," said university student Mohamed Salad.

It was not all peaceful. At least three people, including a pregnant woman, were injured after security forces opened fire while trying to disperse protesters marching toward the attack site, said police Capt. Mohammed Hussein.

Analysts have suggested that al-Shabab, an al-Qaida ally, may have avoided taking responsibility because it did not want to be blamed for the deaths of so many civilians.

A detailed description of the attack emerged. According to a Somali intelligence official investigating the blast, an overloaded truck covered with a tarpaulin approached a security checkpoint outside Mogadishu early Saturday.

The truck, covered in dust, aroused the suspicions of soldiers who ordered the driver to park and get out. The driver, a man who soldiers said behaved in a friendly manner, made a phone call to someone in the capital.

The driver passed the phone to the soldiers to speak to a well-known man who vouched for the truck and persuaded soldiers to allow it into the city, the Somali intelligence official told The Associated Press.

Once through the checkpoint, the truck began to speed along the sandy, potholed road and raced through another checkpoint where soldiers opened fire and flattened one of its tires.

The driver continued before stopping on a busy street and detonating. The blast leveled nearly all nearby buildings in one of Mogadishu's most crowded areas.

The man who vouched for the truck has been arrested and is being held in jail, the Somali intelligence official said.

The massive bomb, weighing between 600 kilograms and 800 kilograms (1,300 pounds and 1,700 pounds), was meant for Mogadishu's heavily fortified international airport, according to security officials. Several countries' embassies are located there.

The driver probably decided to detonate on the street instead because several checkpoints still lay ahead, the Somali intelligence official said.

"Another reason that he would not proceed further is the fact that security forces were coming after it," the official said, speaking on condition of anonymity because he was not authorized to speak to reporters.

The truck bomber had an accomplice driving a smaller car, a Toyota Noah minivan packed with explosives that took another route, said another Somali intelligence official who spoke on condition of anonymity because he was not authorized to speak to reporters. Security forces stopped the vehicle at a checkpoint near the airport, forcing the driver to park and get out.

As soldiers questioned the driver, the minivan detonated, the official said.

The minivan's driver is currently in a prison in Mogadishu, said a senior Somali police officer, Capt. Mohamed Hussein.

Somalia last year saw its highest-ever number of attacks from improvised explosives, at least 395, up from about 265 the year before, according to a threat assessment by the Nairobi-based Sahan research group. Since 2013, when there were 33 such attacks, the threat has grown quickly.

Al-Shabab's capacity to produce and transport ever-larger explosives is improving, the assessment said. Vehicle-borne improvised explosive devices have increased from between 100 and 200 kilograms in 2015 to between 800 and 1,000 kilograms in 2016.

In the ruins of the latest attack, Suban Hussein, the mother of a missing 19-year-old university student, pointed at a large chunk of debris.

"I have searched everywhere else," she declared. "I believe my son's body is under here." No one came to her assistance.

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Associated Press writer Rodney Muhumuza in Kampala, Uganda contributed.